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SubjectRe: [PATCH v3 03/13] seccomp_filters: new mode with configurable syscall filters
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On Thu, Jun 2, 2011 at 2:42 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> On Thu, Jun 02, 2011 at 01:14:54PM -0500, Will Drewry wrote:
>> On Thu, Jun 2, 2011 at 12:36 PM, Paul E. McKenney
>> <paulmck@linux.vnet.ibm.com> wrote:
>> > On Tue, May 31, 2011 at 10:10:35PM -0500, Will Drewry wrote:
>> >> This change adds a new seccomp mode which specifies the allowed system
>> >> calls dynamically.  When in the new mode (2), all system calls are
>> >> checked against process-defined filters - first by system call number,
>> >> then by a filter string.  If an entry exists for a given system call and
>> >> all filter predicates evaluate to true, then the task may proceed.
>> >> Otherwise, the task is killed.
>> >
>> > A few questions below -- I can't say that I understand the RCU usage.
>> >
>> >                                                        Thanx, Paul
>> >
>> >> Filter string parsing and evaluation is handled by the ftrace filter
>> >> engine.  Related patches tweak to the perf filter trace and free
>> >> allowing the calls to be shared. Filters inherit their understanding of
>> >> types and arguments for each system call from the CONFIG_FTRACE_SYSCALLS
>> >> subsystem which already populates this information in syscall_metadata
>> >> associated enter_event (and exit_event) structures. If
>> >> CONFIG_FTRACE_SYSCALLS is not compiled in, only filter strings of "1"
>> >> will be allowed.
>> >>
>> >> The net result is a process may have its system calls filtered using the
>> >> ftrace filter engine's inherent understanding of systems calls.  The set
>> >> of filters is specified through the PR_SET_SECCOMP_FILTER argument in
>> >> prctl(). For example, a filterset for a process, like pdftotext, that
>> >> should only process read-only input could (roughly) look like:
>> >>   sprintf(rdonly, "flags == %u", O_RDONLY|O_LARGEFILE);
>> >>   prctl(PR_SET_SECCOMP_FILTER, __NR_open, rdonly);
>> >>   prctl(PR_SET_SECCOMP_FILTER, __NR__llseek, "1");
>> >>   prctl(PR_SET_SECCOMP_FILTER, __NR_brk, "1");
>> >>   prctl(PR_SET_SECCOMP_FILTER, __NR_close, "1");
>> >>   prctl(PR_SET_SECCOMP_FILTER, __NR_exit_group, "1");
>> >>   prctl(PR_SET_SECCOMP_FILTER, __NR_fstat64, "1");
>> >>   prctl(PR_SET_SECCOMP_FILTER, __NR_mmap2, "1");
>> >>   prctl(PR_SET_SECCOMP_FILTER, __NR_munmap, "1");
>> >>   prctl(PR_SET_SECCOMP_FILTER, __NR_read, "1");
>> >>   prctl(PR_SET_SECCOMP_FILTER, __NR_write, "(fd == 1 | fd == 2)");
>> >>   prctl(PR_SET_SECCOMP, 2);
>> >>
>> >> Subsequent calls to PR_SET_SECCOMP_FILTER for the same system call will
>> >> be &&'d together to ensure that attack surface may only be reduced:
>> >>   prctl(PR_SET_SECCOMP_FILTER, __NR_write, "fd != 2");
>> >>
>> >> With the earlier example, the active filter becomes:
>> >>   "(fd == 1 || fd == 2) && fd != 2"
>> >>
>> >> The patch also adds PR_CLEAR_SECCOMP_FILTER and PR_GET_SECCOMP_FILTER.
>> >> The latter returns the current filter for a system call to userspace:
>> >>
>> >>   prctl(PR_GET_SECCOMP_FILTER, __NR_write, buf, bufsize);
>> >>
>> >> while the former clears any filters for a given system call changing it
>> >> back to a defaulty deny:
>> >>
>> >>   prctl(PR_CLEAR_SECCOMP_FILTER, __NR_write);
>> >>
>> >> v3: - always block execve calls (as per linus torvalds)
>> >>     - add __NR_seccomp_execve(_32) to seccomp-supporting arches
>> >>     - ensure compat tasks can't reach ftrace:syscalls
>> >>     - dropped new defines for seccomp modes.
>> >>     - two level array instead of hlists (sugg. by olof johansson)
>> >>     - added generic Kconfig entry that is not connected.
>> >>     - dropped internal seccomp.h
>> >>     - move prctl helpers to seccomp_filter
>> >>     - killed seccomp_t typedef (as per checkpatch)
>> >> v2: - changed to use the existing syscall number ABI.
>> >>     - prctl changes to minimize parsing in the kernel:
>> >>       prctl(PR_SET_SECCOMP, {0 | 1 | 2 }, { 0 | ON_EXEC });
>> >>       prctl(PR_SET_SECCOMP_FILTER, __NR_read, "fd == 5");
>> >>       prctl(PR_CLEAR_SECCOMP_FILTER, __NR_read);
>> >>       prctl(PR_GET_SECCOMP_FILTER, __NR_read, buf, bufsize);
>> >>     - defined PR_SECCOMP_MODE_STRICT and ..._FILTER
>> >>     - added flags
>> >>     - provide a default fail syscall_nr_to_meta in ftrace
>> >>     - provides fallback for unhooked system calls
>> >>     - use -ENOSYS and ERR_PTR(-ENOSYS) for stubbed functionality
>> >>     - added kernel/seccomp.h to share seccomp.c/seccomp_filter.c
>> >>     - moved to a hlist and 4 bit hash of linked lists
>> >>     - added support to operate without CONFIG_FTRACE_SYSCALLS
>> >>     - moved Kconfig support next to SECCOMP
>> >>     - made Kconfig entries dependent on EXPERIMENTAL
>> >>     - added macros to avoid ifdefs from kernel/fork.c
>> >>     - added compat task/filter matching
>> >>     - drop seccomp.h inclusion in sched.h and drop seccomp_t
>> >>     - added Filtering to "show" output
>> >>     - added on_exec state dup'ing when enabling after a fast-path accept.
>> >>
>> >> Signed-off-by: Will Drewry <wad@chromium.org>
>> >> ---
>> >>  include/linux/prctl.h   |    5 +
>> >>  include/linux/sched.h   |    2 +-
>> >>  include/linux/seccomp.h |   98 ++++++-
>> >>  include/trace/syscall.h |    7 +
>> >>  kernel/Makefile         |    3 +
>> >>  kernel/fork.c           |    3 +
>> >>  kernel/seccomp.c        |   38 ++-
>> >>  kernel/seccomp_filter.c |  784 +++++++++++++++++++++++++++++++++++++++++++++++
>> >>  kernel/sys.c            |   13 +-
>> >>  security/Kconfig        |   17 +
>> >>  10 files changed, 954 insertions(+), 16 deletions(-)
>> >>  create mode 100644 kernel/seccomp_filter.c
>> >>
>> >> diff --git a/include/linux/prctl.h b/include/linux/prctl.h
>> >> index a3baeb2..44723ce 100644
>> >> --- a/include/linux/prctl.h
>> >> +++ b/include/linux/prctl.h
>> >> @@ -64,6 +64,11 @@
>> >>  #define PR_GET_SECCOMP       21
>> >>  #define PR_SET_SECCOMP       22
>> >>
>> >> +/* Get/set process seccomp filters */
>> >> +#define PR_GET_SECCOMP_FILTER        35
>> >> +#define PR_SET_SECCOMP_FILTER        36
>> >> +#define PR_CLEAR_SECCOMP_FILTER      37
>> >> +
>> >>  /* Get/set the capability bounding set (as per security/commoncap.c) */
>> >>  #define PR_CAPBSET_READ 23
>> >>  #define PR_CAPBSET_DROP 24
>> >> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> >> index 18d63ce..3f0bc8d 100644
>> >> --- a/include/linux/sched.h
>> >> +++ b/include/linux/sched.h
>> >> @@ -1374,7 +1374,7 @@ struct task_struct {
>> >>       uid_t loginuid;
>> >>       unsigned int sessionid;
>> >>  #endif
>> >> -     seccomp_t seccomp;
>> >> +     struct seccomp_struct seccomp;
>> >>
>> >>  /* Thread group tracking */
>> >>       u32 parent_exec_id;
>> >> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
>> >> index 167c333..f4434ca 100644
>> >> --- a/include/linux/seccomp.h
>> >> +++ b/include/linux/seccomp.h
>> >> @@ -1,13 +1,33 @@
>> >>  #ifndef _LINUX_SECCOMP_H
>> >>  #define _LINUX_SECCOMP_H
>> >>
>> >> +struct seq_file;
>> >>
>> >>  #ifdef CONFIG_SECCOMP
>> >>
>> >> +#include <linux/errno.h>
>> >>  #include <linux/thread_info.h>
>> >> +#include <linux/types.h>
>> >>  #include <asm/seccomp.h>
>> >>
>> >> -typedef struct { int mode; } seccomp_t;
>> >> +struct seccomp_filters;
>> >> +/**
>> >> + * struct seccomp_struct - the state of a seccomp'ed process
>> >> + *
>> >> + * @mode:
>> >> + *     if this is 1, the process is under standard seccomp rules
>> >> + *             is 2, the process is only allowed to make system calls where
>> >> + *                   associated filters evaluate successfully.
>> >> + * @filters: Metadata for filters if using CONFIG_SECCOMP_FILTER.
>> >> + *           filters assignment/use should be RCU-protected and its contents
>> >> + *           should never be modified when attached to a seccomp_struct.
>> >> + */
>> >> +struct seccomp_struct {
>> >> +     uint16_t mode;
>> >> +#ifdef CONFIG_SECCOMP_FILTER
>> >> +     struct seccomp_filters *filters;
>> >> +#endif
>> >> +};
>> >>
>> >>  extern void __secure_computing(int);
>> >>  static inline void secure_computing(int this_syscall)
>> >> @@ -16,15 +36,14 @@ static inline void secure_computing(int this_syscall)
>> >>               __secure_computing(this_syscall);
>> >>  }
>> >>
>> >> -extern long prctl_get_seccomp(void);
>> >>  extern long prctl_set_seccomp(unsigned long);
>> >> +extern long prctl_get_seccomp(void);
>> >>
>> >>  #else /* CONFIG_SECCOMP */
>> >>
>> >>  #include <linux/errno.h>
>> >>
>> >> -typedef struct { } seccomp_t;
>> >> -
>> >> +struct seccomp_struct { };
>> >>  #define secure_computing(x) do { } while (0)
>> >>
>> >>  static inline long prctl_get_seccomp(void)
>> >> @@ -32,11 +51,80 @@ static inline long prctl_get_seccomp(void)
>> >>       return -EINVAL;
>> >>  }
>> >>
>> >> -static inline long prctl_set_seccomp(unsigned long arg2)
>> >> +static inline long prctl_set_seccomp(unsigned long a2);
>> >>  {
>> >>       return -EINVAL;
>> >>  }
>> >>
>> >>  #endif /* CONFIG_SECCOMP */
>> >>
>> >> +#ifdef CONFIG_SECCOMP_FILTER
>> >> +
>> >> +#define inherit_tsk_seccomp(_child, _orig) do { \
>> >> +     _child->seccomp.mode = _orig->seccomp.mode; \
>> >> +     _child->seccomp.filters = get_seccomp_filters(_orig->seccomp.filters); \
>> >> +     } while (0)
>> >> +#define put_tsk_seccomp(_tsk) put_seccomp_filters(_tsk->seccomp.filters)
>> >> +
>> >> +extern int seccomp_show_filters(struct seccomp_filters *filters,
>> >> +                             struct seq_file *);
>> >> +extern long seccomp_set_filter(int, char *);
>> >> +extern long seccomp_clear_filter(int);
>> >> +extern long seccomp_get_filter(int, char *, unsigned long);
>> >> +
>> >> +extern long prctl_set_seccomp_filter(unsigned long, char __user *);
>> >> +extern long prctl_get_seccomp_filter(unsigned long, char __user *,
>> >> +                                  unsigned long);
>> >> +extern long prctl_clear_seccomp_filter(unsigned long);
>> >> +
>> >> +extern struct seccomp_filters *get_seccomp_filters(struct seccomp_filters *);
>> >> +extern void put_seccomp_filters(struct seccomp_filters *);
>> >> +
>> >> +extern int seccomp_test_filters(int);
>> >> +extern void seccomp_filter_log_failure(int);
>> >> +
>> >> +#else  /* CONFIG_SECCOMP_FILTER */
>> >> +
>> >> +struct seccomp_filters { };
>> >> +#define inherit_tsk_seccomp(_child, _orig) do { } while (0)
>> >> +#define put_tsk_seccomp(_tsk) do { } while (0)
>> >> +
>> >> +static inline int seccomp_show_filters(struct seccomp_filters *filters,
>> >> +                                    struct seq_file *m)
>> >> +{
>> >> +     return -ENOSYS;
>> >> +}
>> >> +
>> >> +static inline long seccomp_set_filter(int syscall_nr, char *filter)
>> >> +{
>> >> +     return -ENOSYS;
>> >> +}
>> >> +
>> >> +static inline long seccomp_clear_filter(int syscall_nr)
>> >> +{
>> >> +     return -ENOSYS;
>> >> +}
>> >> +
>> >> +static inline long seccomp_get_filter(int syscall_nr,
>> >> +                                   char *buf, unsigned long available)
>> >> +{
>> >> +     return -ENOSYS;
>> >> +}
>> >> +
>> >> +static inline long prctl_set_seccomp_filter(unsigned long a2, char __user *a3)
>> >> +{
>> >> +     return -ENOSYS;
>> >> +}
>> >> +
>> >> +static inline long prctl_clear_seccomp_filter(unsigned long a2)
>> >> +{
>> >> +     return -ENOSYS;
>> >> +}
>> >> +
>> >> +static inline long prctl_get_seccomp_filter(unsigned long a2, char __user *a3,
>> >> +                                         unsigned long a4)
>> >> +{
>> >> +     return -ENOSYS;
>> >> +}
>> >> +#endif  /* CONFIG_SECCOMP_FILTER */
>> >>  #endif /* _LINUX_SECCOMP_H */
>> >> diff --git a/include/trace/syscall.h b/include/trace/syscall.h
>> >> index 242ae04..e061ad0 100644
>> >> --- a/include/trace/syscall.h
>> >> +++ b/include/trace/syscall.h
>> >> @@ -35,6 +35,8 @@ struct syscall_metadata {
>> >>  extern unsigned long arch_syscall_addr(int nr);
>> >>  extern int init_syscall_trace(struct ftrace_event_call *call);
>> >>
>> >> +extern struct syscall_metadata *syscall_nr_to_meta(int);
>> >> +
>> >>  extern int reg_event_syscall_enter(struct ftrace_event_call *call);
>> >>  extern void unreg_event_syscall_enter(struct ftrace_event_call *call);
>> >>  extern int reg_event_syscall_exit(struct ftrace_event_call *call);
>> >> @@ -49,6 +51,11 @@ enum print_line_t print_syscall_enter(struct trace_iterator *iter, int flags,
>> >>                                     struct trace_event *event);
>> >>  enum print_line_t print_syscall_exit(struct trace_iterator *iter, int flags,
>> >>                                    struct trace_event *event);
>> >> +#else
>> >> +static inline struct syscall_metadata *syscall_nr_to_meta(int nr)
>> >> +{
>> >> +     return NULL;
>> >> +}
>> >>  #endif
>> >>
>> >>  #ifdef CONFIG_PERF_EVENTS
>> >> diff --git a/kernel/Makefile b/kernel/Makefile
>> >> index 85cbfb3..84e7dfb 100644
>> >> --- a/kernel/Makefile
>> >> +++ b/kernel/Makefile
>> >> @@ -81,6 +81,9 @@ obj-$(CONFIG_DETECT_HUNG_TASK) += hung_task.o
>> >>  obj-$(CONFIG_LOCKUP_DETECTOR) += watchdog.o
>> >>  obj-$(CONFIG_GENERIC_HARDIRQS) += irq/
>> >>  obj-$(CONFIG_SECCOMP) += seccomp.o
>> >> +ifeq ($(CONFIG_SECCOMP_FILTER),y)
>> >> +obj-$(CONFIG_SECCOMP) += seccomp_filter.o
>> >> +endif
>> >>  obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
>> >>  obj-$(CONFIG_TREE_RCU) += rcutree.o
>> >>  obj-$(CONFIG_TREE_PREEMPT_RCU) += rcutree.o
>> >> diff --git a/kernel/fork.c b/kernel/fork.c
>> >> index e7548de..6f835e0 100644
>> >> --- a/kernel/fork.c
>> >> +++ b/kernel/fork.c
>> >> @@ -34,6 +34,7 @@
>> >>  #include <linux/cgroup.h>
>> >>  #include <linux/security.h>
>> >>  #include <linux/hugetlb.h>
>> >> +#include <linux/seccomp.h>
>> >>  #include <linux/swap.h>
>> >>  #include <linux/syscalls.h>
>> >>  #include <linux/jiffies.h>
>> >> @@ -169,6 +170,7 @@ void free_task(struct task_struct *tsk)
>> >>       free_thread_info(tsk->stack);
>> >>       rt_mutex_debug_task_free(tsk);
>> >>       ftrace_graph_exit_task(tsk);
>> >> +     put_tsk_seccomp(tsk);
>> >>       free_task_struct(tsk);
>> >>  }
>> >>  EXPORT_SYMBOL(free_task);
>> >> @@ -280,6 +282,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
>> >>       if (err)
>> >>               goto out;
>> >>
>> >> +     inherit_tsk_seccomp(tsk, orig);
>> >>       setup_thread_stack(tsk, orig);
>> >>       clear_user_return_notifier(tsk);
>> >>       clear_tsk_need_resched(tsk);
>> >> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
>> >> index 57d4b13..0a942be 100644
>> >> --- a/kernel/seccomp.c
>> >> +++ b/kernel/seccomp.c
>> >> @@ -2,16 +2,20 @@
>> >>   * linux/kernel/seccomp.c
>> >>   *
>> >>   * Copyright 2004-2005  Andrea Arcangeli <andrea@cpushare.com>
>> >> + * Copyright (C) 2011 The Chromium OS Authors <chromium-os-dev@chromium.org>
>> >>   *
>> >>   * This defines a simple but solid secure-computing mode.
>> >>   */
>> >>
>> >>  #include <linux/seccomp.h>
>> >>  #include <linux/sched.h>
>> >> +#include <linux/slab.h>
>> >>  #include <linux/compat.h>
>> >> +#include <linux/unistd.h>
>> >> +#include <linux/ftrace_event.h>
>> >>
>> >> +#define SECCOMP_MAX_FILTER_LENGTH MAX_FILTER_STR_VAL
>> >>  /* #define SECCOMP_DEBUG 1 */
>> >> -#define NR_SECCOMP_MODES 1
>> >>
>> >>  /*
>> >>   * Secure computing mode 1 allows only read/write/exit/sigreturn.
>> >> @@ -32,10 +36,9 @@ static int mode1_syscalls_32[] = {
>> >>
>> >>  void __secure_computing(int this_syscall)
>> >>  {
>> >> -     int mode = current->seccomp.mode;
>> >>       int * syscall;
>> >>
>> >> -     switch (mode) {
>> >> +     switch (current->seccomp.mode) {
>> >>       case 1:
>> >>               syscall = mode1_syscalls;
>> >>  #ifdef CONFIG_COMPAT
>> >> @@ -47,6 +50,17 @@ void __secure_computing(int this_syscall)
>> >>                               return;
>> >>               } while (*++syscall);
>> >>               break;
>> >> +#ifdef CONFIG_SECCOMP_FILTER
>> >> +     case 2:
>> >> +             if (this_syscall >= NR_syscalls || this_syscall < 0)
>> >> +                     break;
>> >> +
>> >> +             if (!seccomp_test_filters(this_syscall))
>> >> +                     return;
>> >> +
>> >> +             seccomp_filter_log_failure(this_syscall);
>> >> +             break;
>> >> +#endif
>> >>       default:
>> >>               BUG();
>> >>       }
>> >> @@ -71,16 +85,22 @@ long prctl_set_seccomp(unsigned long seccomp_mode)
>> >>       if (unlikely(current->seccomp.mode))
>> >>               goto out;
>> >>
>> >> -     ret = -EINVAL;
>> >> -     if (seccomp_mode && seccomp_mode <= NR_SECCOMP_MODES) {
>> >> -             current->seccomp.mode = seccomp_mode;
>> >> -             set_thread_flag(TIF_SECCOMP);
>> >> +     ret = 0;
>> >> +     switch (seccomp_mode) {
>> >> +     case 1:
>> >>  #ifdef TIF_NOTSC
>> >>               disable_TSC();
>> >>  #endif
>> >> -             ret = 0;
>> >> +#ifdef CONFIG_SECCOMP_FILTER
>> >> +     case 2:
>> >> +#endif
>> >> +             current->seccomp.mode = seccomp_mode;
>> >> +             set_thread_flag(TIF_SECCOMP);
>> >> +             break;
>> >> +     default:
>> >> +             ret = -EINVAL;
>> >>       }
>> >>
>> >> - out:
>> >> +out:
>> >>       return ret;
>> >>  }
>> >> diff --git a/kernel/seccomp_filter.c b/kernel/seccomp_filter.c
>> >> new file mode 100644
>> >> index 0000000..9782f25
>> >> --- /dev/null
>> >> +++ b/kernel/seccomp_filter.c
>> >> @@ -0,0 +1,784 @@
>> >> +/* filter engine-based seccomp system call filtering
>> >> + *
>> >> + * This program is free software; you can redistribute it and/or modify
>> >> + * it under the terms of the GNU General Public License as published by
>> >> + * the Free Software Foundation; either version 2 of the License, or
>> >> + * (at your option) any later version.
>> >> + *
>> >> + * This program is distributed in the hope that it will be useful,
>> >> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>> >> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>> >> + * GNU General Public License for more details.
>> >> + *
>> >> + * You should have received a copy of the GNU General Public License
>> >> + * along with this program; if not, write to the Free Software
>> >> + * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
>> >> + *
>> >> + * Copyright (C) 2011 The Chromium OS Authors <chromium-os-dev@chromium.org>
>> >> + */
>> >> +
>> >> +#include <linux/compat.h>
>> >> +#include <linux/err.h>
>> >> +#include <linux/errno.h>
>> >> +#include <linux/ftrace_event.h>
>> >> +#include <linux/seccomp.h>
>> >> +#include <linux/seq_file.h>
>> >> +#include <linux/sched.h>
>> >> +#include <linux/slab.h>
>> >> +#include <linux/uaccess.h>
>> >> +
>> >> +#include <asm/syscall.h>
>> >> +#include <trace/syscall.h>
>> >> +
>> >> +
>> >> +#define SECCOMP_MAX_FILTER_LENGTH MAX_FILTER_STR_VAL
>> >> +
>> >> +#define SECCOMP_FILTER_ALLOW "1"
>> >> +#define SECCOMP_ACTION_DENY 0xffff
>> >> +#define SECCOMP_ACTION_ALLOW 0xfffe
>> >> +
>> >> +/**
>> >> + * struct seccomp_filters - container for seccomp filterset
>> >> + *
>> >> + * @syscalls: array of 16-bit indices into @event_filters by syscall_nr
>> >> + *            May also be SECCOMP_ACTION_DENY or SECCOMP_ACTION_ALLOW
>> >> + * @event_filters: array of pointers to ftrace event objects
>> >> + * @count: size of @event_filters
>> >> + * @flags: anonymous struct to wrap filters-specific flags
>> >> + * @usage: reference count to simplify use.
>> >> + */
>> >> +struct seccomp_filters {
>> >> +     uint16_t syscalls[NR_syscalls];
>> >> +     struct event_filter **event_filters;
>> >> +     uint16_t count;
>> >> +     struct {
>> >> +             uint32_t compat:1,
>> >> +                      __reserved:31;
>> >> +     } flags;
>> >> +     atomic_t usage;
>> >> +};
>> >> +
>> >> +/* Handle ftrace symbol non-existence */
>> >> +#ifdef CONFIG_FTRACE_SYSCALLS
>> >> +#define create_event_filter(_ef_pptr, _event_type, _str) \
>> >> +     ftrace_parse_filter(_ef_pptr, _event_type, _str)
>> >> +#define get_filter_string(_ef) ftrace_get_filter_string(_ef)
>> >> +#define free_event_filter(_f) ftrace_free_filter(_f)
>> >> +
>> >> +#else
>> >> +
>> >> +#define create_event_filter(_ef_pptr, _event_type, _str) (-ENOSYS)
>> >> +#define get_filter_string(_ef) (NULL)
>> >> +#define free_event_filter(_f) do { } while (0)
>> >> +#endif
>> >> +
>> >> +/**
>> >> + * seccomp_filters_new - allocates a new filters object
>> >> + * @count: count to allocate for the event_filters array
>> >> + *
>> >> + * Returns ERR_PTR on error or an allocated object.
>> >> + */
>> >> +static struct seccomp_filters *seccomp_filters_new(uint16_t count)
>> >> +{
>> >> +     struct seccomp_filters *f;
>> >> +
>> >> +     if (count >= SECCOMP_ACTION_ALLOW)
>> >> +             return ERR_PTR(-EINVAL);
>> >> +
>> >> +     f = kzalloc(sizeof(struct seccomp_filters), GFP_KERNEL);
>> >> +     if (!f)
>> >> +             return ERR_PTR(-ENOMEM);
>> >> +
>> >> +     /* Lazy SECCOMP_ACTION_DENY assignment. */
>> >> +     memset(f->syscalls, 0xff, sizeof(f->syscalls));
>> >> +     atomic_set(&f->usage, 1);
>> >> +
>> >> +     f->event_filters = NULL;
>> >> +     f->count = count;
>> >> +     if (!count)
>> >> +             return f;
>> >> +
>> >> +     f->event_filters = kzalloc(count * sizeof(struct event_filter *),
>> >> +                                GFP_KERNEL);
>> >> +     if (!f->event_filters) {
>> >> +             kfree(f);
>> >> +             f = ERR_PTR(-ENOMEM);
>> >> +     }
>> >> +     return f;
>> >> +}
>> >> +
>> >> +/**
>> >> + * seccomp_filters_free - cleans up the filter list and frees the table
>> >> + * @filters: NULL or live object to be completely destructed.
>> >> + */
>> >> +static void seccomp_filters_free(struct seccomp_filters *filters)
>> >> +{
>> >> +     uint16_t count = 0;
>> >> +     if (!filters)
>> >> +             return;
>> >> +     while (count < filters->count) {
>> >> +             struct event_filter *f = filters->event_filters[count];
>> >> +             free_event_filter(f);
>> >> +             count++;
>> >> +     }
>> >> +     kfree(filters->event_filters);
>> >> +     kfree(filters);
>> >> +}
>> >> +
>> >> +static void __put_seccomp_filters(struct seccomp_filters *orig)
>> >> +{
>> >> +     WARN_ON(atomic_read(&orig->usage));
>> >> +     seccomp_filters_free(orig);
>> >> +}
>> >> +
>> >> +#define seccomp_filter_allow(_id) ((_id) == SECCOMP_ACTION_ALLOW)
>> >> +#define seccomp_filter_deny(_id) ((_id) == SECCOMP_ACTION_DENY)
>> >> +#define seccomp_filter_dynamic(_id) \
>> >> +     (!seccomp_filter_allow(_id) && !seccomp_filter_deny(_id))
>> >> +static inline uint16_t seccomp_filter_id(const struct seccomp_filters *f,
>> >> +                                      int syscall_nr)
>> >> +{
>> >> +     if (!f)
>> >> +             return SECCOMP_ACTION_DENY;
>> >> +     return f->syscalls[syscall_nr];
>> >> +}
>> >> +
>> >> +static inline struct event_filter *seccomp_dynamic_filter(
>> >> +             const struct seccomp_filters *filters, uint16_t id)
>> >> +{
>> >> +     if (!seccomp_filter_dynamic(id))
>> >> +             return NULL;
>> >> +     return filters->event_filters[id];
>> >> +}
>> >> +
>> >> +static inline void set_seccomp_filter_id(struct seccomp_filters *filters,
>> >> +                                      int syscall_nr, uint16_t id)
>> >> +{
>> >> +     filters->syscalls[syscall_nr] = id;
>> >> +}
>> >> +
>> >> +static inline void set_seccomp_filter(struct seccomp_filters *filters,
>> >> +                                   int syscall_nr, uint16_t id,
>> >> +                                   struct event_filter *dynamic_filter)
>> >> +{
>> >> +     filters->syscalls[syscall_nr] = id;
>> >> +     if (seccomp_filter_dynamic(id))
>> >> +             filters->event_filters[id] = dynamic_filter;
>> >> +}
>> >> +
>> >> +static struct event_filter *alloc_event_filter(int syscall_nr,
>> >> +                                            const char *filter_string)
>> >> +{
>> >> +     struct syscall_metadata *data;
>> >> +     struct event_filter *filter = NULL;
>> >> +     int err;
>> >> +
>> >> +     data = syscall_nr_to_meta(syscall_nr);
>> >> +     /* Argument-based filtering only works on ftrace-hooked syscalls. */
>> >> +     err = -ENOSYS;
>> >> +     if (!data)
>> >> +             goto fail;
>> >> +     err = create_event_filter(&filter,
>> >> +                               data->enter_event->event.type,
>> >> +                               filter_string);
>> >> +     if (err)
>> >> +             goto fail;
>> >> +
>> >> +     return filter;
>> >> +fail:
>> >> +     kfree(filter);
>> >> +     return ERR_PTR(err);
>> >> +}
>> >> +
>> >> +/**
>> >> + * seccomp_filters_copy - copies filters from src to dst.
>> >> + *
>> >> + * @dst: seccomp_filters to populate.
>> >> + * @src: table to read from.
>> >> + * @skip: specifies an entry, by system call, to skip.
>> >> + *
>> >> + * Returns non-zero on failure.
>> >> + * Both the source and the destination should have no simultaneous
>> >> + * writers, and dst should be exclusive to the caller.
>> >> + * If @skip is < 0, it is ignored.
>> >> + */
>> >> +static int seccomp_filters_copy(struct seccomp_filters *dst,
>> >> +                             const struct seccomp_filters *src,
>> >> +                             int skip)
>> >> +{
>> >> +     int id = 0, ret = 0, nr;
>> >> +     memcpy(&dst->flags, &src->flags, sizeof(src->flags));
>> >> +     memcpy(dst->syscalls, src->syscalls, sizeof(dst->syscalls));
>> >> +     if (!src->count)
>> >> +             goto done;
>> >> +     for (nr = 0; nr < NR_syscalls; ++nr) {
>> >> +             struct event_filter *filter;
>> >> +             const char *str;
>> >> +             uint16_t src_id = seccomp_filter_id(src, nr);
>> >> +             if (nr == skip) {
>> >> +                     set_seccomp_filter(dst, nr, SECCOMP_ACTION_DENY,
>> >> +                                        NULL);
>> >> +                     continue;
>> >> +             }
>> >> +             if (!seccomp_filter_dynamic(src_id))
>> >> +                     continue;
>> >> +             if (id >= dst->count) {
>> >> +                     ret = -EINVAL;
>> >> +                     goto done;
>> >> +             }
>> >> +             str = get_filter_string(seccomp_dynamic_filter(src, src_id));
>> >> +             filter = alloc_event_filter(nr, str);
>> >> +             if (IS_ERR(filter)) {
>> >> +                     ret = PTR_ERR(filter);
>> >> +                     goto done;
>> >> +             }
>> >> +             set_seccomp_filter(dst, nr, id, filter);
>> >> +             id++;
>> >> +     }
>> >> +
>> >> +done:
>> >> +     return ret;
>> >> +}
>> >> +
>> >> +/**
>> >> + * seccomp_extend_filter - appends more text to a syscall_nr's filter
>> >> + * @filters: unattached filter object to operate on
>> >> + * @syscall_nr: syscall number to update filters for
>> >> + * @filter_string: string to append to the existing filter
>> >> + *
>> >> + * The new string will be &&'d to the original filter string to ensure that it
>> >> + * always matches the existing predicates or less:
>> >> + *   (old_filter) && @filter_string
>> >> + * A new seccomp_filters instance is returned on success and a ERR_PTR on
>> >> + * failure.
>> >> + */
>> >> +static int seccomp_extend_filter(struct seccomp_filters *filters,
>> >> +                              int syscall_nr, char *filter_string)
>> >> +{
>> >> +     struct event_filter *filter;
>> >> +     uint16_t id = seccomp_filter_id(filters, syscall_nr);
>> >> +     char *merged = NULL;
>> >> +     int ret = -EINVAL, expected;
>> >> +
>> >> +     /* No extending with a "1". */
>> >> +     if (!strcmp(SECCOMP_FILTER_ALLOW, filter_string))
>> >> +             goto out;
>> >> +
>> >> +     filter = seccomp_dynamic_filter(filters, id);
>> >> +     ret = -ENOENT;
>> >> +     if (!filter)
>> >> +             goto out;
>> >> +
>> >> +     merged = kzalloc(SECCOMP_MAX_FILTER_LENGTH + 1, GFP_KERNEL);
>> >> +     ret = -ENOMEM;
>> >> +     if (!merged)
>> >> +             goto out;
>> >> +
>> >> +     expected = snprintf(merged, SECCOMP_MAX_FILTER_LENGTH, "(%s) && %s",
>> >> +                         get_filter_string(filter), filter_string);
>> >> +     ret = -E2BIG;
>> >> +     if (expected >= SECCOMP_MAX_FILTER_LENGTH || expected < 0)
>> >> +             goto out;
>> >> +
>> >> +     /* Free the old filter */
>> >> +     free_event_filter(filter);
>> >> +     set_seccomp_filter(filters, syscall_nr, id, NULL);
>> >> +
>> >> +     /* Replace it */
>> >> +     filter = alloc_event_filter(syscall_nr, merged);
>> >> +     if (IS_ERR(filter)) {
>> >> +             ret = PTR_ERR(filter);
>> >> +             goto out;
>> >> +     }
>> >> +     set_seccomp_filter(filters, syscall_nr, id, filter);
>> >> +     ret = 0;
>> >> +
>> >> +out:
>> >> +     kfree(merged);
>> >> +     return ret;
>> >> +}
>> >> +
>> >> +/**
>> >> + * seccomp_add_filter - adds a filter for an unfiltered syscall
>> >> + * @filters: filters object to add a filter/action to
>> >> + * @syscall_nr: system call number to add a filter for
>> >> + * @filter_string: the filter string to apply
>> >> + *
>> >> + * Returns 0 on success and non-zero otherwise.
>> >> + */
>> >> +static int seccomp_add_filter(struct seccomp_filters *filters, int syscall_nr,
>> >> +                           char *filter_string)
>> >> +{
>> >> +     struct event_filter *filter;
>> >> +     int ret = 0;
>> >> +
>> >> +     if (!strcmp(SECCOMP_FILTER_ALLOW, filter_string)) {
>> >> +             set_seccomp_filter(filters, syscall_nr,
>> >> +                                SECCOMP_ACTION_ALLOW, NULL);
>> >> +             goto out;
>> >> +     }
>> >> +
>> >> +     filter = alloc_event_filter(syscall_nr, filter_string);
>> >> +     if (IS_ERR(filter)) {
>> >> +             ret = PTR_ERR(filter);
>> >> +             goto out;
>> >> +     }
>> >> +     /* Always add to the last slot available since additions are
>> >> +      * are only done one at a time.
>> >> +      */
>> >> +     set_seccomp_filter(filters, syscall_nr, filters->count - 1, filter);
>> >> +out:
>> >> +     return ret;
>> >> +}
>> >> +
>> >> +/* Wrap optional ftrace syscall support. Returns 1 on match or 0 otherwise. */
>> >> +static int filter_match_current(struct event_filter *event_filter)
>> >> +{
>> >> +     int err = 0;
>> >> +#ifdef CONFIG_FTRACE_SYSCALLS
>> >> +     uint8_t syscall_state[64];
>> >> +
>> >> +     memset(syscall_state, 0, sizeof(syscall_state));
>> >> +
>> >> +     /* The generic tracing entry can remain zeroed. */
>> >> +     err = ftrace_syscall_enter_state(syscall_state, sizeof(syscall_state),
>> >> +                                      NULL);
>> >> +     if (err)
>> >> +             return 0;
>> >> +
>> >> +     err = filter_match_preds(event_filter, syscall_state);
>> >> +#endif
>> >> +     return err;
>> >> +}
>> >> +
>> >> +static const char *syscall_nr_to_name(int syscall)
>> >> +{
>> >> +     const char *syscall_name = "unknown";
>> >> +     struct syscall_metadata *data = syscall_nr_to_meta(syscall);
>> >> +     if (data)
>> >> +             syscall_name = data->name;
>> >> +     return syscall_name;
>> >> +}
>> >> +
>> >> +static void filters_set_compat(struct seccomp_filters *filters)
>> >> +{
>> >> +#ifdef CONFIG_COMPAT
>> >> +     if (is_compat_task())
>> >> +             filters->flags.compat = 1;
>> >> +#endif
>> >> +}
>> >> +
>> >> +static inline int filters_compat_mismatch(struct seccomp_filters *filters)
>> >> +{
>> >> +     int ret = 0;
>> >> +     if (!filters)
>> >> +             return 0;
>> >> +#ifdef CONFIG_COMPAT
>> >> +     if (!!(is_compat_task()) == filters->flags.compat)
>> >> +             ret = 1;
>> >> +#endif
>> >> +     return ret;
>> >> +}
>> >> +
>> >> +static inline int syscall_is_execve(int syscall)
>> >> +{
>> >> +     int nr = __NR_execve;
>> >> +#ifdef CONFIG_COMPAT
>> >> +     if (is_compat_task())
>> >> +             nr = __NR_seccomp_execve_32;
>> >> +#endif
>> >> +     return syscall == nr;
>> >> +}
>> >> +
>> >> +#ifndef KSTK_EIP
>> >> +#define KSTK_EIP(x) 0L
>> >> +#endif
>> >> +
>> >> +void seccomp_filter_log_failure(int syscall)
>> >> +{
>> >> +     pr_info("%s[%d]: system call %d (%s) blocked at 0x%lx\n",
>> >> +             current->comm, task_pid_nr(current), syscall,
>> >> +             syscall_nr_to_name(syscall), KSTK_EIP(current));
>> >> +}
>> >> +
>> >> +/* put_seccomp_state - decrements the reference count of @orig and may free. */
>> >> +void put_seccomp_filters(struct seccomp_filters *orig)
>> >> +{
>> >> +     if (!orig)
>> >> +             return;
>> >> +
>> >> +     if (atomic_dec_and_test(&orig->usage))
>> >> +             __put_seccomp_filters(orig);
>> >> +}
>> >> +
>> >> +/* get_seccomp_state - increments the reference count of @orig */
>> >> +struct seccomp_filters *get_seccomp_filters(struct seccomp_filters *orig)
>> >
>> > Nit: the name does not match the comment.
>>
>> Will fix it here and above. Thanks!
>>
>> >> +{
>> >> +     if (!orig)
>> >> +             return NULL;
>> >> +     atomic_inc(&orig->usage);
>> >> +     return orig;
>> >
>> > This is called in an RCU read-side critical section.  What exactly is
>> > RCU protecting?  I would expect an rcu_dereference() or one of the
>> > RCU list-traversal primitives somewhere, either here or at the caller.
>>
>> Ah, I spaced on rcu_dereference().  The goal was to make the
>> assignment and replacement of the seccomp_filters pointer
>> RCU-protected (in seccomp_state) so there's no concern over it being
>> replaced partial on platforms where pointer assignments are non-atomic
>> - such as via /proc/<pid>/seccomp_filters access or a call via the
>> exported symbols.  Object lifetime is managed by reference counting so
>> that I don't have to worry about extending the RCU read-side critical
>> section by much or deal with pre-allocations.
>>
>> I'll add rcu_dereference() to all the get_seccomp_filters() uses where
>> it makes sense, so that it is called safely.  Just to make sure, does
>> it make sense to continue to rcu protect the specific pointer?
>
> It might.  The usual other options is to use a lock outside of the element
> containing the reference count to protect reference-count manipulation.
> If there is some convenient lock, especially if it is already held where
> needed, then locking is more straightforward.  Otherwise, RCU is usually
> a reasonable option.

I was concerned about the overhead a lock would have at each system
call entry, but I didn't benchmark it to see. I'll add the
rcu_dereference right away, then look into seeing whether there's a
cleaner approach. I was trying to be overly protective of mutating
any data internal to the filters through complete replacement on any
change. I'll take a step back and see if

>> >> +}
>> >> +
>> >> +/**
>> >> + * seccomp_test_filters - tests 'current' against the given syscall
>> >> + * @state: seccomp_state of current to use.
>> >> + * @syscall: number of the system call to test
>> >> + *
>> >> + * Returns 0 on ok and non-zero on error/failure.
>> >> + */
>> >> +int seccomp_test_filters(int syscall)
>> >> +{
>> >> +     uint16_t id;
>> >> +     struct event_filter *filter;
>> >> +     struct seccomp_filters *filters;
>> >> +     int ret = -EACCES;
>> >> +
>> >> +     rcu_read_lock();
>> >> +     filters = get_seccomp_filters(current->seccomp.filters);
>> >> +     rcu_read_unlock();
>> >> +
>> >> +     if (!filters)
>> >> +             goto out;
>> >> +
>> >> +     if (filters_compat_mismatch(filters)) {
>> >> +             pr_info("%s[%d]: seccomp_filter compat() mismatch.\n",
>> >> +                     current->comm, task_pid_nr(current));
>> >> +             goto out;
>> >> +     }
>> >> +
>> >> +     /* execve is never allowed. */
>> >> +     if (syscall_is_execve(syscall))
>> >> +             goto out;
>> >> +
>> >> +     ret = 0;
>> >> +     id = seccomp_filter_id(filters, syscall);
>> >> +     if (seccomp_filter_allow(id))
>> >> +             goto out;
>> >> +
>> >> +     ret = -EACCES;
>> >> +     if (!seccomp_filter_dynamic(id))
>> >> +             goto out;
>> >> +
>> >> +     filter = seccomp_dynamic_filter(filters, id);
>> >> +     if (filter && filter_match_current(filter))
>> >> +             ret = 0;
>> >> +out:
>> >> +     put_seccomp_filters(filters);
>> >> +     return ret;
>> >> +}
>> >> +
>> >> +/**
>> >> + * seccomp_show_filters - prints the current filter state to a seq_file
>> >> + * @filters: properly get()'d filters object
>> >> + * @m: the prepared seq_file to receive the data
>> >> + *
>> >> + * Returns 0 on a successful write.
>> >> + */
>> >> +int seccomp_show_filters(struct seccomp_filters *filters, struct seq_file *m)
>> >> +{
>> >> +     int syscall;
>> >> +     seq_printf(m, "Mode: %d\n", current->seccomp.mode);
>> >> +     if (!filters)
>> >> +             goto out;
>> >> +
>> >> +     for (syscall = 0; syscall < NR_syscalls; ++syscall) {
>> >> +             uint16_t id = seccomp_filter_id(filters, syscall);
>> >> +             const char *filter_string = SECCOMP_FILTER_ALLOW;
>> >> +             if (seccomp_filter_deny(id))
>> >> +                     continue;
>> >> +             seq_printf(m, "%d (%s): ",
>> >> +                           syscall,
>> >> +                           syscall_nr_to_name(syscall));
>> >> +             if (seccomp_filter_dynamic(id))
>> >> +                     filter_string = get_filter_string(
>> >> +                                       seccomp_dynamic_filter(filters, id));
>> >> +             seq_printf(m, "%s\n", filter_string);
>> >> +     }
>> >> +out:
>> >> +     return 0;
>> >> +}
>> >> +EXPORT_SYMBOL_GPL(seccomp_show_filters);
>> >> +
>> >> +/**
>> >> + * seccomp_get_filter - copies the filter_string into "buf"
>> >> + * @syscall_nr: system call number to look up
>> >> + * @buf: destination buffer
>> >> + * @bufsize: available space in the buffer.
>> >> + *
>> >> + * Context: User context only. This function may sleep on allocation and
>> >> + *          operates on current. current must be attempting a system call
>> >> + *          when this is called.
>> >> + *
>> >> + * Looks up the filter for the given system call number on current.  If found,
>> >> + * the string length of the NUL-terminated buffer is returned and < 0 is
>> >> + * returned on error. The NUL byte is not included in the length.
>> >> + */
>> >> +long seccomp_get_filter(int syscall_nr, char *buf, unsigned long bufsize)
>> >> +{
>> >> +     struct seccomp_filters *filters;
>> >> +     struct event_filter *filter;
>> >> +     long ret = -EINVAL;
>> >> +     uint16_t id;
>> >> +
>> >> +     if (bufsize > SECCOMP_MAX_FILTER_LENGTH)
>> >> +             bufsize = SECCOMP_MAX_FILTER_LENGTH;
>> >> +
>> >> +     rcu_read_lock();
>> >> +     filters = get_seccomp_filters(current->seccomp.filters);
>> >> +     rcu_read_unlock();
>> >> +
>> >> +     if (!filters)
>> >> +             goto out;
>> >> +
>> >> +     ret = -ENOENT;
>> >> +     id = seccomp_filter_id(filters, syscall_nr);
>> >> +     if (seccomp_filter_deny(id))
>> >> +             goto out;
>> >> +
>> >> +     if (seccomp_filter_allow(id)) {
>> >> +             ret = strlcpy(buf, SECCOMP_FILTER_ALLOW, bufsize);
>> >> +             goto copied;
>> >> +     }
>> >> +
>> >> +     filter = seccomp_dynamic_filter(filters, id);
>> >> +     if (!filter)
>> >> +             goto out;
>> >> +     ret = strlcpy(buf, get_filter_string(filter), bufsize);
>> >> +
>> >> +copied:
>> >> +     if (ret >= bufsize) {
>> >> +             ret = -ENOSPC;
>> >> +             goto out;
>> >> +     }
>> >> +     /* Zero out any remaining buffer, just in case. */
>> >> +     memset(buf + ret, 0, bufsize - ret);
>> >> +out:
>> >> +     put_seccomp_filters(filters);
>> >> +     return ret;
>> >> +}
>> >> +EXPORT_SYMBOL_GPL(seccomp_get_filter);
>> >> +
>> >> +/**
>> >> + * seccomp_clear_filter: clears the seccomp filter for a syscall.
>> >> + * @syscall_nr: the system call number to clear filters for.
>> >> + *
>> >> + * Context: User context only. This function may sleep on allocation and
>> >> + *          operates on current. current must be attempting a system call
>> >> + *          when this is called.
>> >> + *
>> >> + * Returns 0 on success.
>> >> + */
>> >> +long seccomp_clear_filter(int syscall_nr)
>> >> +{
>> >> +     struct seccomp_filters *filters = NULL, *orig_filters;
>> >> +     uint16_t id;
>> >> +     int ret = -EINVAL;
>> >> +
>> >> +     rcu_read_lock();
>> >> +     orig_filters = get_seccomp_filters(current->seccomp.filters);
>> >> +     rcu_read_unlock();
>> >> +
>> >> +     if (!orig_filters)
>> >> +             goto out;
>> >> +
>> >> +     if (filters_compat_mismatch(orig_filters))
>> >> +             goto out;
>> >> +
>> >> +     id = seccomp_filter_id(orig_filters, syscall_nr);
>> >> +     if (seccomp_filter_deny(id))
>> >> +             goto out;
>> >> +
>> >> +     /* Create a new filters object for the task */
>> >> +     if (seccomp_filter_dynamic(id))
>> >> +             filters = seccomp_filters_new(orig_filters->count - 1);
>> >> +     else
>> >> +             filters = seccomp_filters_new(orig_filters->count);
>> >> +
>> >> +     if (IS_ERR(filters)) {
>> >> +             ret = PTR_ERR(filters);
>> >> +             goto out;
>> >> +     }
>> >> +
>> >> +     /* Copy, but drop the requested entry. */
>> >> +     ret = seccomp_filters_copy(filters, orig_filters, syscall_nr);
>> >> +     if (ret)
>> >> +             goto out;
>> >> +     get_seccomp_filters(filters);  /* simplify the out: path */
>> >> +
>> >> +     rcu_assign_pointer(current->seccomp.filters, filters);
>> >
>> > What prevents two copies of seccomp_clear_filter() from running
>> > concurrently?
>>
>> Nothing - the last one wins assignment, but the objects themselves
>> should be internally consistent to the parallel calls.  If that's a
>> concern, a per-task writer mutex could be used just to ensure
>> simultaneous calls to clear and set are performed serially.  Would
>> that make more sense?
>
> Here is the sequence of events that I am concerned about:
>
> o       CPU 0 sets orig_filters to point to the current filters.
>
> o       CPU 1 sets its local orig_filters to point to the current
>        set of filters.
>
> o       Both CPUs allocate new filters and use rcu_assign_pointer()
>        to do the update.  As you say, the last one wins, but it appears
>        to me that the first one leaks memory.
>
> o       Both CPUs free the object referenced by their orig_filters,
>        which might or might not result in a double free, depending
>        on exactly what happens below.  (You might actually be OK,
>        I didn't check -- leaking memory was enough for me to call
>        attention to this.)
>
> So yes, please use some kind of mutual exclusion.  Not sure what you
> mean by "per-task mutex", but whatever it is must prevent two different
> tasks from acting on the same set of filters at the same time.  The
> thing that I call "per-task mutex" would -not- do that.

Ah nice. Yeah that would most definitely leak as there would be a
remaining increment for the task that isn't dropped.

Since those interfaces acquire the filter itself from
current->seccomp.filters, I was thinking a mutex in current, e.g.,
current->seccomp.write_mutex, would fit the bill to ensure that
pointer isn't accessed for replacement. I'll look at this and the rcu
usage to see if I'm taking the most logical approach. I pretty much
always get locking wrong in some way and perhaps I can simplify
further and get the correct guarantees. I really appreciate the keen
observation and explanation!

>> >> +     synchronize_rcu();
>> >> +     put_seccomp_filters(orig_filters);  /* for the task */
>> >> +out:
>> >> +     put_seccomp_filters(orig_filters);  /* for the get */
>> >> +     put_seccomp_filters(filters);  /* for the extra get */
>> >> +     return ret;
>> >> +}
>> >> +EXPORT_SYMBOL_GPL(seccomp_clear_filter);
>> >> +
>> >> +/**
>> >> + * seccomp_set_filter: - Adds/extends a seccomp filter for a syscall.
>> >> + * @syscall_nr: system call number to apply the filter to.
>> >> + * @filter: ftrace filter string to apply.
>> >> + *
>> >> + * Context: User context only. This function may sleep on allocation and
>> >> + *          operates on current. current must be attempting a system call
>> >> + *          when this is called.
>> >> + *
>> >> + * New filters may be added for system calls when the current task is
>> >> + * not in a secure computing mode (seccomp).  Otherwise, existing filters may
>> >> + * be extended.
>> >> + *
>> >> + * Returns 0 on success or an errno on failure.
>> >> + */
>> >> +long seccomp_set_filter(int syscall_nr, char *filter)
>> >> +{
>> >> +     struct seccomp_filters *filters = NULL, *orig_filters = NULL;
>> >> +     uint16_t id;
>> >> +     long ret = -EINVAL;
>> >> +     uint16_t filters_needed;
>> >> +
>> >> +     if (!filter)
>> >> +             goto out;
>> >> +
>> >> +     filter = strstrip(filter);
>> >> +     /* Disallow empty strings. */
>> >> +     if (filter[0] == 0)
>> >> +             goto out;
>> >> +
>> >> +     rcu_read_lock();
>> >> +     orig_filters = get_seccomp_filters(current->seccomp.filters);
>> >> +     rcu_read_unlock();
>> >> +
>> >> +     /* After the first call, compatibility mode is selected permanently. */
>> >> +     ret = -EACCES;
>> >> +     if (filters_compat_mismatch(orig_filters))
>> >> +             goto out;
>> >> +
>> >> +     filters_needed = orig_filters ? orig_filters->count : 0;
>> >> +     id = seccomp_filter_id(orig_filters, syscall_nr);
>> >> +     if (seccomp_filter_deny(id)) {
>> >> +             /* Don't allow DENYs to be changed when in a seccomp mode */
>> >> +             ret = -EACCES;
>> >> +             if (current->seccomp.mode)
>> >> +                     goto out;
>> >> +             filters_needed++;
>> >> +     }
>> >> +
>> >> +     filters = seccomp_filters_new(filters_needed);
>> >> +     if (IS_ERR(filters)) {
>> >> +             ret = PTR_ERR(filters);
>> >> +             goto out;
>> >> +     }
>> >> +
>> >> +     filters_set_compat(filters);
>> >> +     if (orig_filters) {
>> >> +             ret = seccomp_filters_copy(filters, orig_filters, -1);
>> >> +             if (ret)
>> >> +                     goto out;
>> >> +     }
>> >> +
>> >> +     if (seccomp_filter_deny(id))
>> >> +             ret = seccomp_add_filter(filters, syscall_nr, filter);
>> >> +     else
>> >> +             ret = seccomp_extend_filter(filters, syscall_nr, filter);
>> >> +     if (ret)
>> >> +             goto out;
>> >> +     get_seccomp_filters(filters);  /* simplify the error paths */
>> >> +
>> >> +     rcu_assign_pointer(current->seccomp.filters, filters);
>> >
>> > Again, what prevents two copies of seccomp_set_filter() from running
>> > concurrently?
>>
>> Same deal - nothing, but I'd be happy to add a guard if it makes sense.
>>
>> Thanks!
>>
>> >> +     synchronize_rcu();
>> >> +     put_seccomp_filters(orig_filters);  /* for the task */
>> >> +out:
>> >> +     put_seccomp_filters(orig_filters);  /* for the get */
>> >> +     put_seccomp_filters(filters);  /* for get or task, on err */
>> >> +     return ret;
>> >> +}
>> >> +EXPORT_SYMBOL_GPL(seccomp_set_filter);
>> >> +
>> >> +long prctl_set_seccomp_filter(unsigned long syscall_nr,
>> >> +                           char __user *user_filter)
>> >> +{
>> >> +     int nr;
>> >> +     long ret;
>> >> +     char *filter = NULL;
>> >> +
>> >> +     ret = -EINVAL;
>> >> +     if (syscall_nr >= NR_syscalls)
>> >> +             goto out;
>> >> +
>> >> +     ret = -EFAULT;
>> >> +     if (!user_filter)
>> >> +             goto out;
>> >> +
>> >> +     filter = kzalloc(SECCOMP_MAX_FILTER_LENGTH + 1, GFP_KERNEL);
>> >> +     ret = -ENOMEM;
>> >> +     if (!filter)
>> >> +             goto out;
>> >> +
>> >> +     ret = -EFAULT;
>> >> +     if (strncpy_from_user(filter, user_filter,
>> >> +                           SECCOMP_MAX_FILTER_LENGTH - 1) < 0)
>> >> +             goto out;
>> >> +
>> >> +     nr = (int) syscall_nr;
>> >> +     ret = seccomp_set_filter(nr, filter);
>> >> +
>> >> +out:
>> >> +     kfree(filter);
>> >> +     return ret;
>> >> +}
>> >> +
>> >> +long prctl_clear_seccomp_filter(unsigned long syscall_nr)
>> >> +{
>> >> +     int nr = -1;
>> >> +     long ret;
>> >> +
>> >> +     ret = -EINVAL;
>> >> +     if (syscall_nr >= NR_syscalls)
>> >> +             goto out;
>> >> +
>> >> +     nr = (int) syscall_nr;
>> >> +     ret = seccomp_clear_filter(nr);
>> >> +
>> >> +out:
>> >> +     return ret;
>> >> +}
>> >> +
>> >> +long prctl_get_seccomp_filter(unsigned long syscall_nr, char __user *dst,
>> >> +                           unsigned long available)
>> >> +{
>> >> +     int ret, nr;
>> >> +     unsigned long copied;
>> >> +     char *buf = NULL;
>> >> +     ret = -EINVAL;
>> >> +     if (!available)
>> >> +             goto out;
>> >> +     /* Ignore extra buffer space. */
>> >> +     if (available > SECCOMP_MAX_FILTER_LENGTH)
>> >> +             available = SECCOMP_MAX_FILTER_LENGTH;
>> >> +
>> >> +     ret = -EINVAL;
>> >> +     if (syscall_nr >= NR_syscalls)
>> >> +             goto out;
>> >> +     nr = (int) syscall_nr;
>> >> +
>> >> +     ret = -ENOMEM;
>> >> +     buf = kmalloc(available, GFP_KERNEL);
>> >> +     if (!buf)
>> >> +             goto out;
>> >> +
>> >> +     ret = seccomp_get_filter(nr, buf, available);
>> >> +     if (ret < 0)
>> >> +             goto out;
>> >> +
>> >> +     /* Include the NUL byte in the copy. */
>> >> +     copied = copy_to_user(dst, buf, ret + 1);
>> >> +     ret = -ENOSPC;
>> >> +     if (copied)
>> >> +             goto out;
>> >> +     ret = 0;
>> >> +out:
>> >> +     kfree(buf);
>> >> +     return ret;
>> >> +}
>> >> diff --git a/kernel/sys.c b/kernel/sys.c
>> >> index af468ed..ed60d06 100644
>> >> --- a/kernel/sys.c
>> >> +++ b/kernel/sys.c
>> >> @@ -1698,13 +1698,24 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>> >>               case PR_SET_ENDIAN:
>> >>                       error = SET_ENDIAN(me, arg2);
>> >>                       break;
>> >> -
>> >>               case PR_GET_SECCOMP:
>> >>                       error = prctl_get_seccomp();
>> >>                       break;
>> >>               case PR_SET_SECCOMP:
>> >>                       error = prctl_set_seccomp(arg2);
>> >>                       break;
>> >> +             case PR_SET_SECCOMP_FILTER:
>> >> +                     error = prctl_set_seccomp_filter(arg2,
>> >> +                                                      (char __user *) arg3);
>> >> +                     break;
>> >> +             case PR_CLEAR_SECCOMP_FILTER:
>> >> +                     error = prctl_clear_seccomp_filter(arg2);
>> >> +                     break;
>> >> +             case PR_GET_SECCOMP_FILTER:
>> >> +                     error = prctl_get_seccomp_filter(arg2,
>> >> +                                                      (char __user *) arg3,
>> >> +                                                      arg4);
>> >> +                     break;
>> >>               case PR_GET_TSC:
>> >>                       error = GET_TSC_CTL(arg2);
>> >>                       break;
>> >> diff --git a/security/Kconfig b/security/Kconfig
>> >> index 95accd4..c76adf2 100644
>> >> --- a/security/Kconfig
>> >> +++ b/security/Kconfig
>> >> @@ -2,6 +2,10 @@
>> >>  # Security configuration
>> >>  #
>> >>
>> >> +# Make seccomp filter Kconfig switch below available
>> >> +config HAVE_SECCOMP_FILTER
>> >> +       bool
>> >> +
>> >>  menu "Security options"
>> >>
>> >>  config KEYS
>> >> @@ -82,6 +86,19 @@ config SECURITY_DMESG_RESTRICT
>> >>
>> >>         If you are unsure how to answer this question, answer N.
>> >>
>> >> +config SECCOMP_FILTER
>> >> +     bool "Enable seccomp-based system call filtering"
>> >> +     select SECCOMP
>> >> +     depends on HAVE_SECCOMP_FILTER && EXPERIMENTAL
>> >> +     help
>> >> +       This kernel feature expands CONFIG_SECCOMP to allow computing
>> >> +       in environments with reduced kernel access dictated by the
>> >> +       application itself through prctl calls.  If
>> >> +       CONFIG_FTRACE_SYSCALLS is available, then system call
>> >> +       argument-based filtering predicates may be used.
>> >> +
>> >> +       See Documentation/prctl/seccomp_filter.txt for more detail.
>> >> +
>> >>  config SECURITY
>> >>       bool "Enable different security models"
>> >>       depends on SYSFS
>> >> --
>> >> 1.7.0.4
>> >>
>> >> --
>> >> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>> >> the body of a message to majordomo@vger.kernel.org
>> >> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>> >> Please read the FAQ at  http://www.tux.org/lkml/
>> >
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>> the body of a message to majordomo@vger.kernel.org
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>> Please read the FAQ at  http://www.tux.org/lkml/
>
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