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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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