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

Paul E. McKenney
2011-06-02T19:42:37Z

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.

> >> +}
> >> +
> >> +/**
> >> + * 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.

> >> +     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
> >>
> >> --
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> >> Please read the FAQ at  http://www.tux.org/lkml/
> >
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