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    SubjectRe: [PATCH v3 03/13] seccomp_filters: new mode with configurable syscall filters
    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.

    > +{
    > + 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.

    > +}
    > +
    > +/**
    > + * 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?

    > + 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?

    > + 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
    >
    > --
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