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

[Paul E. McKenney]

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@xxxxxxxxxxxx>
> ---
>  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@xxxxxxxxxxxx>
> + * Copyright (C) 2011 The Chromium OS Authors <chromium-os-dev@xxxxxxxxxxxx>
>   *
>   * 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@xxxxxxxxxxxx>
> + */
> +
> +#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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