Message-ID: <20120112085316.GB16518@peqn>
Date:	Thu, 12 Jan 2012 09:53:16 +0100
From:	Serge Hallyn <serge.hallyn@...onical.com>
To:	Will Drewry <wad@...omium.org>
Cc:	linux-kernel@...r.kernel.org, keescook@...omium.org,
	john.johansen@...onical.com, coreyb@...ux.vnet.ibm.com,
	pmoore@...hat.com, eparis@...hat.com, djm@...drot.org,
	torvalds@...ux-foundation.org, segoon@...nwall.com,
	rostedt@...dmis.org, jmorris@...ei.org, scarybeasts@...il.com,
	avi@...hat.com, penberg@...helsinki.fi, viro@...iv.linux.org.uk,
	luto@....edu, mingo@...e.hu, akpm@...ux-foundation.org,
	khilman@...com, borislav.petkov@....com, amwang@...hat.com,
	oleg@...hat.com, ak@...ux.intel.com, eric.dumazet@...il.com,
	gregkh@...e.de, dhowells@...hat.com, daniel.lezcano@...e.fr,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, olofj@...omium.org,
	mhalcrow@...gle.com, dlaor@...hat.com
Subject: Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF

Quoting Will Drewry (wad@...omium.org):
> This patch adds support for seccomp mode 2.  This mode enables dynamic
> enforcement of system call filtering policy in the kernel as specified
> by a userland task.  The policy is expressed in terms of a BPF program,
> as is used for userland-exposed socket filtering.  Instead of network
> data, the BPF program is evaluated over struct user_regs_struct at the
> time of the system call (as retrieved using regviews).
> 
> A filter program may be installed by a userland task by calling
>   prctl(PR_ATTACH_SECCOMP_FILTER, &fprog);
> where fprog is of type struct sock_fprog.
> 
> If the first filter program allows subsequent prctl(2) calls, then
> additional filter programs may be attached.  All attached programs
> must be evaluated before a system call will be allowed to proceed.
> 
> To avoid CONFIG_COMPAT related landmines, once a filter program is
> installed using specific is_compat_task() and current->personality, it
> is not allowed to make system calls or attach additional filters which
> use a different combination of is_compat_task() and
> current->personality.
> 
> Filter programs may _only_ cross the execve(2) barrier if last filter
> program was attached by a task with CAP_SYS_ADMIN capabilities in its
> user namespace.  Once a task-local filter program is attached from a
> process without privileges, execve will fail.  This ensures that only
> privileged parent task can affect its privileged children (e.g., setuid
> binary).
> 
> There are a number of benefits to this approach. A few of which are
> as follows:
> - BPF has been exposed to userland for a long time.
> - Userland already knows its ABI: expected register layout and system
>   call numbers.
> - Full register information is provided which may be relevant for
>   certain syscalls (fork, rt_sigreturn) or for other userland
>   filtering tactics (checking the PC).
> - No time-of-check-time-of-use vulnerable data accesses are possible.
> 
> This patch includes its own BPF evaluator, but relies on the
> net/core/filter.c BPF checking code.  It is possible to share
> evaluators, but the performance sensitive nature of the network
> filtering path makes it an iterative optimization which (I think :) can
> be tackled separately via separate patchsets. (And at some point sharing
> BPF JIT code!)
> 
> Signed-off-by: Will Drewry <wad@...omium.org>

Hey Will,

A few comments below, but otherwise

Acked-by: Serge Hallyn <serge.hallyn@...onical.com>

thanks,
-serge

> ---
>  fs/exec.c               |    5 +
>  include/linux/prctl.h   |    3 +
>  include/linux/seccomp.h |   70 +++++-
>  kernel/Makefile         |    1 +
>  kernel/fork.c           |    4 +
>  kernel/seccomp.c        |    8 +
>  kernel/seccomp_filter.c |  639 +++++++++++++++++++++++++++++++++++++++++++++++
>  kernel/sys.c            |    4 +
>  security/Kconfig        |   12 +
>  9 files changed, 743 insertions(+), 3 deletions(-)
>  create mode 100644 kernel/seccomp_filter.c
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 3625464..e9cc89c 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -44,6 +44,7 @@
>  #include <linux/namei.h>
>  #include <linux/mount.h>
>  #include <linux/security.h>
> +#include <linux/seccomp.h>
>  #include <linux/syscalls.h>
>  #include <linux/tsacct_kern.h>
>  #include <linux/cn_proc.h>
> @@ -1477,6 +1478,10 @@ static int do_execve_common(const char *filename,
>  	if (retval)
>  		goto out_ret;
>  
> +	retval = seccomp_check_exec();
> +	if (retval)
> +		goto out_ret;
> +
>  	retval = -ENOMEM;
>  	bprm = kzalloc(sizeof(*bprm), GFP_KERNEL);
>  	if (!bprm)
> diff --git a/include/linux/prctl.h b/include/linux/prctl.h
> index a3baeb2..15e2460 100644
> --- a/include/linux/prctl.h
> +++ b/include/linux/prctl.h
> @@ -64,6 +64,9 @@
>  #define PR_GET_SECCOMP	21
>  #define PR_SET_SECCOMP	22
>  
> +/* Set process seccomp filters */
> +#define PR_ATTACH_SECCOMP_FILTER	36
> +
>  /* 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/seccomp.h b/include/linux/seccomp.h
> index cc7a4e9..99d163e 100644
> --- a/include/linux/seccomp.h
> +++ b/include/linux/seccomp.h
> @@ -5,9 +5,28 @@
>  #ifdef CONFIG_SECCOMP
>  
>  #include <linux/thread_info.h>
> +#include <linux/types.h>
>  #include <asm/seccomp.h>
>  
> -typedef struct { int mode; } seccomp_t;
> +struct seccomp_filter;
> +/**
> + * struct seccomp_struct - the state of a seccomp'ed process
> + *
> + * @mode:
> + *     if this is 0, seccomp is not in use.
> + *             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.
> + * @filter: Metadata for filter if using CONFIG_SECCOMP_FILTER.
> + *          @filter must only be accessed from the context of current as there
> + *          is no guard.
> + */
> +typedef struct seccomp_struct {
> +	int mode;
> +#ifdef CONFIG_SECCOMP_FILTER
> +	struct seccomp_filter *filter;
> +#endif
> +} seccomp_t;
>  
>  extern void __secure_computing(int);
>  static inline void secure_computing(int this_syscall)
> @@ -28,8 +47,7 @@ static inline int seccomp_mode(seccomp_t *s)
>  
>  #include <linux/errno.h>
>  
> -typedef struct { } seccomp_t;
> -
> +typedef struct seccomp_struct { } seccomp_t;
>  #define secure_computing(x) do { } while (0)
>  
>  static inline long prctl_get_seccomp(void)
> @@ -49,4 +67,50 @@ static inline int seccomp_mode(seccomp_t *s)
>  
>  #endif /* CONFIG_SECCOMP */
>  
> +#ifdef CONFIG_SECCOMP_FILTER
> +
> +#define seccomp_filter_init_task(_tsk) do { \
> +	(_tsk)->seccomp.filter = NULL; \
> +} while (0);
> +
> +/* No locking is needed here because the task_struct will
> + * have no parallel consumers.
> + */
> +#define seccomp_filter_free_task(_tsk) do { \
> +	put_seccomp_filter((_tsk)->seccomp.filter); \
> +} while (0);
> +
> +extern int seccomp_check_exec(void);
> +
> +extern long prctl_attach_seccomp_filter(char __user *);
> +
> +extern struct seccomp_filter *get_seccomp_filter(struct seccomp_filter *);
> +extern void put_seccomp_filter(struct seccomp_filter *);
> +
> +extern int seccomp_test_filters(int);
> +extern void seccomp_filter_log_failure(int);
> +extern void seccomp_filter_fork(struct task_struct *child,
> +				struct task_struct *parent);
> +
> +#else  /* CONFIG_SECCOMP_FILTER */
> +
> +#include <linux/errno.h>
> +
> +struct seccomp_filter { };
> +#define seccomp_filter_init_task(_tsk) do { } while (0);
> +#define seccomp_filter_fork(_tsk, _orig) do { } while (0);
> +#define seccomp_filter_free_task(_tsk) do { } while (0);
> +
> +static inline int seccomp_check_exec(void)
> +{
> +	return 0;
> +}
> +
> +
> +static inline long prctl_attach_seccomp_filter(char __user *a2)
> +{
> +	return -ENOSYS;
> +}
> +
> +#endif  /* CONFIG_SECCOMP_FILTER */
>  #endif /* _LINUX_SECCOMP_H */
> diff --git a/kernel/Makefile b/kernel/Makefile
> index e898c5b..0584090 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -79,6 +79,7 @@ obj-$(CONFIG_DETECT_HUNG_TASK) += hung_task.o
>  obj-$(CONFIG_LOCKUP_DETECTOR) += watchdog.o
>  obj-$(CONFIG_GENERIC_HARDIRQS) += irq/
>  obj-$(CONFIG_SECCOMP) += seccomp.o
> +obj-$(CONFIG_SECCOMP_FILTER) += seccomp_filter.o
>  obj-$(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 da4a6a1..cc1d628 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>
> @@ -166,6 +167,7 @@ void free_task(struct task_struct *tsk)
>  	free_thread_info(tsk->stack);
>  	rt_mutex_debug_task_free(tsk);
>  	ftrace_graph_exit_task(tsk);
> +	seccomp_filter_free_task(tsk);
>  	free_task_struct(tsk);
>  }
>  EXPORT_SYMBOL(free_task);
> @@ -1209,6 +1211,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>  	/* Perform scheduler related setup. Assign this task to a CPU. */
>  	sched_fork(p);
>  
> +	seccomp_filter_init_task(p);
>  	retval = perf_event_init_task(p);
>  	if (retval)
>  		goto bad_fork_cleanup_policy;
> @@ -1375,6 +1378,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>  	if (clone_flags & CLONE_THREAD)
>  		threadgroup_fork_read_unlock(current);
>  	perf_event_fork(p);
> +	seccomp_filter_fork(p, current);
>  	return p;
>  
>  bad_fork_free_pid:
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 57d4b13..78719be 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -47,6 +47,14 @@ void __secure_computing(int this_syscall)
>  				return;
>  		} while (*++syscall);
>  		break;
> +#ifdef CONFIG_SECCOMP_FILTER
> +	case 2:
> +		if (seccomp_test_filters(this_syscall) == 0)
> +			return;
> +
> +		seccomp_filter_log_failure(this_syscall);
> +		break;
> +#endif
>  	default:
>  		BUG();
>  	}
> diff --git a/kernel/seccomp_filter.c b/kernel/seccomp_filter.c
> new file mode 100644
> index 0000000..4770847
> --- /dev/null
> +++ b/kernel/seccomp_filter.c
> @@ -0,0 +1,639 @@
> +/* bpf program-based 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@...omium.org>
> + */
> +
> +#include <linux/capability.h>
> +#include <linux/compat.h>
> +#include <linux/err.h>
> +#include <linux/errno.h>
> +#include <linux/rculist.h>
> +#include <linux/filter.h>
> +#include <linux/kallsyms.h>
> +#include <linux/kref.h>
> +#include <linux/module.h>
> +#include <linux/pid.h>
> +#include <linux/prctl.h>
> +#include <linux/ptrace.h>
> +#include <linux/ratelimit.h>
> +#include <linux/reciprocal_div.h>
> +#include <linux/regset.h>
> +#include <linux/seccomp.h>
> +#include <linux/security.h>
> +#include <linux/sched.h>
> +#include <linux/slab.h>
> +#include <linux/uaccess.h>
> +#include <linux/user.h>
> +
> +
> +/**
> + * struct seccomp_filter - container for seccomp BPF programs
> + *
> + * @usage: reference count to manage the object lifetime.
> + *         get/put helpers should be used when accessing an instance
> + *         outside of a lifetime-guarded section.  In general, this
> + *         is only needed for handling filters shared across tasks.
> + * @creator: pointer to the pid that created this filter
> + * @parent: pointer to the ancestor which this filter will be composed with.
> + * @flags: provide information about filter from creation time.
> + * @personality: personality of the process at filter creation time.
> + * @insns: the BPF program instructions to evaluate
> + * @count: the number of instructions in the program.
> + *
> + * seccomp_filter objects should never be modified after being attached
> + * to a task_struct (other than @usage).
> + */
> +struct seccomp_filter {
> +	struct kref usage;
> +	struct pid *creator;
> +	struct seccomp_filter *parent;
> +	struct {
> +		uint32_t admin:1,  /* can allow execve */
> +			 compat:1,  /* CONFIG_COMPAT */
> +			 __reserved:30;
> +	} flags;
> +	int personality;
> +	unsigned short count;  /* Instruction count */
> +	struct sock_filter insns[0];
> +};
> +
> +static unsigned int seccomp_run_filter(const u8 *buf,
> +				       const size_t buflen,
> +				       const struct sock_filter *);
> +
> +/**
> + * seccomp_filter_alloc - allocates a new filter object
> + * @padding: size of the insns[0] array in bytes
> + *
> + * The @padding should be a multiple of
> + * sizeof(struct sock_filter).
> + *
> + * Returns ERR_PTR on error or an allocated object.
> + */
> +static struct seccomp_filter *seccomp_filter_alloc(unsigned long padding)
> +{
> +	struct seccomp_filter *f;
> +	unsigned long bpf_blocks = padding / sizeof(struct sock_filter);
> +
> +	/* Drop oversized requests. */
> +	if (bpf_blocks == 0 || bpf_blocks > BPF_MAXINSNS)
> +		return ERR_PTR(-EINVAL);
> +
> +	/* Padding should always be in sock_filter increments. */
> +	BUG_ON(padding % sizeof(struct sock_filter));

I still think the BUG_ON here is harsh given that the progsize is passed
in by userspace.  Was there a reason not to return -EINVAL here?

> +
> +	f = kzalloc(sizeof(struct seccomp_filter) + padding, GFP_KERNEL);
> +	if (!f)
> +		return ERR_PTR(-ENOMEM);
> +	kref_init(&f->usage);
> +	f->creator = get_task_pid(current, PIDTYPE_PID);
> +	f->count = bpf_blocks;
> +	return f;
> +}
> +
> +/**
> + * seccomp_filter_free - frees the allocated filter.
> + * @filter: NULL or live object to be completely destructed.
> + */
> +static void seccomp_filter_free(struct seccomp_filter *filter)
> +{
> +	if (!filter)
> +		return;
> +	put_seccomp_filter(filter->parent);
> +	put_pid(filter->creator);
> +	kfree(filter);
> +}
> +
> +static void __put_seccomp_filter(struct kref *kref)
> +{
> +	struct seccomp_filter *orig =
> +		container_of(kref, struct seccomp_filter, usage);
> +	seccomp_filter_free(orig);
> +}
> +
> +void seccomp_filter_log_failure(int syscall)
> +{
> +	pr_info("%s[%d]: system call %d blocked at 0x%lx\n",
> +		current->comm, task_pid_nr(current), syscall,
> +		KSTK_EIP(current));
> +}
> +
> +/* put_seccomp_filter - decrements the ref count of @orig and may free. */
> +void put_seccomp_filter(struct seccomp_filter *orig)
> +{
> +	if (!orig)
> +		return;
> +	kref_put(&orig->usage, __put_seccomp_filter);
> +}
> +
> +/* get_seccomp_filter - increments the reference count of @orig. */
> +struct seccomp_filter *get_seccomp_filter(struct seccomp_filter *orig)
> +{
> +	if (!orig)
> +		return NULL;
> +	kref_get(&orig->usage);
> +	return orig;
> +}
> +
> +static int seccomp_check_personality(struct seccomp_filter *filter)
> +{
> +	if (filter->personality != current->personality)
> +		return -EACCES;
> +#ifdef CONFIG_COMPAT
> +	if (filter->flags.compat != (!!(is_compat_task())))
> +		return -EACCES;
> +#endif
> +	return 0;
> +}
> +
> +static const struct user_regset *
> +find_prstatus(const struct user_regset_view *view)
> +{
> +	const struct user_regset *regset;
> +	int n;
> +
> +	/* Skip 0. */
> +	for (n = 1; n < view->n; ++n) {
> +		regset = view->regsets + n;
> +		if (regset->core_note_type == NT_PRSTATUS)
> +			return regset;
> +	}
> +
> +	return NULL;
> +}
> +
> +/**
> + * seccomp_get_regs - returns a pointer to struct user_regs_struct
> + * @scratch: preallocated storage of size @available
> + * @available: pointer to the size of scratch.
> + *
> + * Returns NULL if the registers cannot be acquired or copied.
> + * Returns a populated pointer to @scratch by default.
> + * Otherwise, returns a pointer to a a u8 array containing the struct
> + * user_regs_struct appropriate for the task personality.  The pointer
> + * may be to the beginning of @scratch or to an externally managed data
> + * structure.  On success, @available should be updated with the
> + * valid region size of the returned pointer.
> + *
> + * If the architecture overrides the linkage, then the pointer may pointer to
> + * another location.
> + */
> +__weak u8 *seccomp_get_regs(u8 *scratch, size_t *available)
> +{
> +	/* regset is usually returned based on task personality, not current
> +	 * system call convention.  This behavior makes it unsafe to execute
> +	 * BPF programs over regviews if is_compat_task or the personality
> +	 * have changed since the program was installed.
> +	 */
> +	const struct user_regset_view *view = task_user_regset_view(current);
> +	const struct user_regset *regset = &view->regsets[0];
> +	size_t scratch_size = *available;
> +	if (regset->core_note_type != NT_PRSTATUS) {
> +		/* The architecture should override this method for speed. */
> +		regset = find_prstatus(view);
> +		if (!regset)
> +			return NULL;
> +	}
> +	*available = regset->n * regset->size;
> +	/* Make sure the scratch space isn't exceeded. */
> +	if (*available > scratch_size)
> +		*available = scratch_size;
> +	if (regset->get(current, regset, 0, *available, scratch, NULL))
> +		return NULL;
> +	return scratch;
> +}
> +
> +/**
> + * seccomp_test_filters - tests 'current' against the given syscall
> + * @syscall: number of the system call to test
> + *
> + * Returns 0 on ok and non-zero on error/failure.
> + */
> +int seccomp_test_filters(int syscall)
> +{
> +	struct seccomp_filter *filter;
> +	u8 regs_tmp[sizeof(struct user_regs_struct)], *regs;
> +	size_t regs_size = sizeof(struct user_regs_struct);
> +	int ret = -EACCES;
> +
> +	filter = current->seccomp.filter; /* uses task ref */
> +	if (!filter)
> +		goto out;
> +
> +	/* All filters in the list are required to share the same system call
> +	 * convention so only the first filter is ever checked.
> +	 */
> +	if (seccomp_check_personality(filter))
> +		goto out;
> +
> +	/* Grab the user_regs_struct.  Normally, regs == &regs_tmp, but
> +	 * that is not mandatory.  E.g., it may return a point to
> +	 * task_pt_regs(current).  NULL checking is mandatory.
> +	 */
> +	regs = seccomp_get_regs(regs_tmp, &regs_size);
> +	if (!regs)
> +		goto out;
> +
> +	/* Only allow a system call if it is allowed in all ancestors. */
> +	ret = 0;
> +	for ( ; filter != NULL; filter = filter->parent) {
> +		/* Allowed if return value is the size of the data supplied. */
> +		if (seccomp_run_filter(regs, regs_size, filter->insns) !=
> +		    regs_size)
> +			ret = -EACCES;
> +	}
> +out:
> +	return ret;
> +}
> +
> +/**
> + * seccomp_attach_filter: Attaches a seccomp filter to current.
> + * @fprog: BPF program to install
> + *
> + * 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 (usually prctl).
> + *
> + * This function may be called repeatedly to install additional filters.
> + * Every filter successfully installed will be evaluated (in reverse order)
> + * for each system call the thread makes.
> + *
> + * Returns 0 on success or an errno on failure.
> + */
> +long seccomp_attach_filter(struct sock_fprog *fprog)
> +{
> +	struct seccomp_filter *filter = NULL;
> +	/* Note, len is a short so overflow should be impossible. */
> +	unsigned long fp_size = fprog->len * sizeof(struct sock_filter);
> +	long ret = -EPERM;
> +
> +	/* Allocate a new seccomp_filter */
> +	filter = seccomp_filter_alloc(fp_size);
> +	if (IS_ERR(filter)) {
> +		ret = PTR_ERR(filter);
> +		goto out;
> +	}
> +
> +	/* Lock the process personality and calling convention. */
> +#ifdef CONFIG_COMPAT
> +	if (is_compat_task())
> +		filter->flags.compat = 1;
> +#endif
> +	filter->personality = current->personality;
> +
> +	/* Auditing is not needed since the capability wasn't requested */
> +	if (security_real_capable_noaudit(current, current_user_ns(),
> +					  CAP_SYS_ADMIN) == 0)
> +		filter->flags.admin = 1;
> +
> +	/* Copy the instructions from fprog. */
> +	ret = -EFAULT;
> +	if (copy_from_user(filter->insns, fprog->filter, fp_size))
> +		goto out;
> +
> +	/* Check the fprog */
> +	ret = sk_chk_filter(filter->insns, filter->count);
> +	if (ret)
> +		goto out;
> +
> +	/* If there is an existing filter, make it the parent
> +	 * and reuse the existing task-based ref.
> +	 */
> +	filter->parent = current->seccomp.filter;
> +
> +	/* Force all filters to use one system call convention. */
> +	ret = -EINVAL;
> +	if (filter->parent) {
> +		if (filter->parent->flags.compat != filter->flags.compat)
> +			goto out;
> +		if (filter->parent->personality != filter->personality)
> +			goto out;
> +	}
> +
> +	/* Double claim the new filter so we can release it below simplifying
> +	 * the error paths earlier.
> +	 */
> +	ret = 0;
> +	get_seccomp_filter(filter);
> +	current->seccomp.filter = filter;
> +	/* Engage seccomp if it wasn't. This doesn't use PR_SET_SECCOMP. */
> +	if (!current->seccomp.mode) {
> +		current->seccomp.mode = 2;
> +		set_thread_flag(TIF_SECCOMP);
> +	}
> +
> +out:
> +	put_seccomp_filter(filter);  /* for get or task, on err */
> +	return ret;
> +}
> +
> +long prctl_attach_seccomp_filter(char __user *user_filter)
> +{
> +	struct sock_fprog fprog;
> +	long ret = -EINVAL;
> +
> +	ret = -EFAULT;
> +	if (!user_filter)
> +		goto out;
> +
> +	if (copy_from_user(&fprog, user_filter, sizeof(fprog)))
> +		goto out;
> +
> +	ret = seccomp_attach_filter(&fprog);
> +out:
> +	return ret;
> +}
> +
> +/**
> + * seccomp_check_exec: determines if exec is allowed for current
> + * Returns 0 if allowed.
> + */
> +int seccomp_check_exec(void)
> +{
> +	if (current->seccomp.mode != 2)
> +		return 0;
> +	/* We can rely on the task refcount for the filter. */
> +	if (!current->seccomp.filter)
> +		return -EPERM;
> +	/* The last attached filter set for the process is checked. It must
> +	 * have been installed with CAP_SYS_ADMIN capabilities.

This comment is confusing.  By 'It must' you mean that if not, it's
denied.  But if I didn't know better I would read that as "we can't
get to this code unless".  Can you change it to something like
"Exec is refused unless the filter was installed with CAP_SYS_ADMIN
privilege"?

> +	 */
> +	if (current->seccomp.filter->flags.admin)
> +		return 0;
> +	return -EPERM;
> +}
> +
> +/* seccomp_filter_fork: manages inheritance on fork
> + * @child: forkee
> + * @parent: forker
> + * Ensures that @child inherit a seccomp_filter iff seccomp is enabled
> + * and the set of filters is marked as 'enabled'.
> + */
> +void seccomp_filter_fork(struct task_struct *child,
> +			 struct task_struct *parent)
> +{
> +	if (!parent->seccomp.mode)
> +		return;
> +	child->seccomp.mode = parent->seccomp.mode;
> +	child->seccomp.filter = get_seccomp_filter(parent->seccomp.filter);
> +}
> +
> +/* Returns a pointer to the BPF evaluator after checking the offset and size
> + * boundaries.  The signature almost matches the signature from
> + * net/core/filter.c with the hopes of sharing code in the future.
> + */
> +static const void *load_pointer(const u8 *buf, size_t buflen,
> +				int offset, size_t size,
> +				void *unused)
> +{
> +	if (offset >= buflen)
> +		goto fail;
> +	if (offset < 0)
> +		goto fail;
> +	if (size > buflen - offset)
> +		goto fail;
> +	return buf + offset;
> +fail:
> +	return NULL;
> +}
> +
> +/**
> + * seccomp_run_filter - evaluate BPF (over user_regs_struct)
> + *	@buf: buffer to execute the filter over
> + *	@buflen: length of the buffer
> + *	@fentry: filter to apply
> + *
> + * Decode and apply filter instructions to the buffer.
> + * Return length to keep, 0 for none. @buf is a regset we are
> + * filtering, @filter is the array of filter instructions.
> + * Because all jumps are guaranteed to be before last instruction,
> + * and last instruction guaranteed to be a RET, we dont need to check
> + * flen.
> + *
> + * See core/net/filter.c as this is nearly an exact copy.
> + * At some point, it would be nice to merge them to take advantage of
> + * optimizations (like JIT).
> + *
> + * A successful filter must return the full length of the data. Anything less
> + * will currently result in a seccomp failure.  In the future, it may be
> + * possible to use that for hard filtering registers on the fly so it is
> + * ideal for consumers to return 0 on intended failure.
> + */
> +static unsigned int seccomp_run_filter(const u8 *buf,
> +				       const size_t buflen,
> +				       const struct sock_filter *fentry)
> +{
> +	const void *ptr;
> +	u32 A = 0;			/* Accumulator */
> +	u32 X = 0;			/* Index Register */
> +	u32 mem[BPF_MEMWORDS];		/* Scratch Memory Store */
> +	u32 tmp;
> +	int k;
> +
> +	/*
> +	 * Process array of filter instructions.
> +	 */
> +	for (;; fentry++) {
> +#if defined(CONFIG_X86_32)
> +#define	K (fentry->k)
> +#else
> +		const u32 K = fentry->k;
> +#endif
> +
> +		switch (fentry->code) {
> +		case BPF_S_ALU_ADD_X:
> +			A += X;
> +			continue;
> +		case BPF_S_ALU_ADD_K:
> +			A += K;
> +			continue;
> +		case BPF_S_ALU_SUB_X:
> +			A -= X;
> +			continue;
> +		case BPF_S_ALU_SUB_K:
> +			A -= K;
> +			continue;
> +		case BPF_S_ALU_MUL_X:
> +			A *= X;
> +			continue;
> +		case BPF_S_ALU_MUL_K:
> +			A *= K;
> +			continue;
> +		case BPF_S_ALU_DIV_X:
> +			if (X == 0)
> +				return 0;
> +			A /= X;
> +			continue;
> +		case BPF_S_ALU_DIV_K:
> +			A = reciprocal_divide(A, K);
> +			continue;
> +		case BPF_S_ALU_AND_X:
> +			A &= X;
> +			continue;
> +		case BPF_S_ALU_AND_K:
> +			A &= K;
> +			continue;
> +		case BPF_S_ALU_OR_X:
> +			A |= X;
> +			continue;
> +		case BPF_S_ALU_OR_K:
> +			A |= K;
> +			continue;
> +		case BPF_S_ALU_LSH_X:
> +			A <<= X;
> +			continue;
> +		case BPF_S_ALU_LSH_K:
> +			A <<= K;
> +			continue;
> +		case BPF_S_ALU_RSH_X:
> +			A >>= X;
> +			continue;
> +		case BPF_S_ALU_RSH_K:
> +			A >>= K;
> +			continue;
> +		case BPF_S_ALU_NEG:
> +			A = -A;
> +			continue;
> +		case BPF_S_JMP_JA:
> +			fentry += K;
> +			continue;
> +		case BPF_S_JMP_JGT_K:
> +			fentry += (A > K) ? fentry->jt : fentry->jf;
> +			continue;
> +		case BPF_S_JMP_JGE_K:
> +			fentry += (A >= K) ? fentry->jt : fentry->jf;
> +			continue;
> +		case BPF_S_JMP_JEQ_K:
> +			fentry += (A == K) ? fentry->jt : fentry->jf;
> +			continue;
> +		case BPF_S_JMP_JSET_K:
> +			fentry += (A & K) ? fentry->jt : fentry->jf;
> +			continue;
> +		case BPF_S_JMP_JGT_X:
> +			fentry += (A > X) ? fentry->jt : fentry->jf;
> +			continue;
> +		case BPF_S_JMP_JGE_X:
> +			fentry += (A >= X) ? fentry->jt : fentry->jf;
> +			continue;
> +		case BPF_S_JMP_JEQ_X:
> +			fentry += (A == X) ? fentry->jt : fentry->jf;
> +			continue;
> +		case BPF_S_JMP_JSET_X:
> +			fentry += (A & X) ? fentry->jt : fentry->jf;
> +			continue;
> +		case BPF_S_LD_W_ABS:
> +			k = K;
> +load_w:
> +			ptr = load_pointer(buf, buflen, k, 4, &tmp);
> +			if (ptr != NULL) {
> +				/* Note, unlike on network data, values are not
> +				 * byte swapped.
> +				 */
> +				A = *(const u32 *)ptr;
> +				continue;
> +			}
> +			return 0;
> +		case BPF_S_LD_H_ABS:
> +			k = K;
> +load_h:
> +			ptr = load_pointer(buf, buflen, k, 2, &tmp);
> +			if (ptr != NULL) {
> +				A = *(const u16 *)ptr;
> +				continue;
> +			}
> +			return 0;
> +		case BPF_S_LD_B_ABS:
> +			k = K;
> +load_b:
> +			ptr = load_pointer(buf, buflen, k, 1, &tmp);
> +			if (ptr != NULL) {
> +				A = *(const u8 *)ptr;
> +				continue;
> +			}
> +			return 0;
> +		case BPF_S_LD_W_LEN:
> +			A = buflen;
> +			continue;
> +		case BPF_S_LDX_W_LEN:
> +			X = buflen;
> +			continue;
> +		case BPF_S_LD_W_IND:
> +			k = X + K;
> +			goto load_w;
> +		case BPF_S_LD_H_IND:
> +			k = X + K;
> +			goto load_h;
> +		case BPF_S_LD_B_IND:
> +			k = X + K;
> +			goto load_b;
> +		case BPF_S_LDX_B_MSH:
> +			ptr = load_pointer(buf, buflen, K, 1, &tmp);
> +			if (ptr != NULL) {
> +				X = (*(u8 *)ptr & 0xf) << 2;
> +				continue;
> +			}
> +			return 0;
> +		case BPF_S_LD_IMM:
> +			A = K;
> +			continue;
> +		case BPF_S_LDX_IMM:
> +			X = K;
> +			continue;
> +		case BPF_S_LD_MEM:
> +			A = mem[K];
> +			continue;
> +		case BPF_S_LDX_MEM:
> +			X = mem[K];
> +			continue;
> +		case BPF_S_MISC_TAX:
> +			X = A;
> +			continue;
> +		case BPF_S_MISC_TXA:
> +			A = X;
> +			continue;
> +		case BPF_S_RET_K:
> +			return K;
> +		case BPF_S_RET_A:
> +			return A;
> +		case BPF_S_ST:
> +			mem[K] = A;
> +			continue;
> +		case BPF_S_STX:
> +			mem[K] = X;
> +			continue;
> +		case BPF_S_ANC_PROTOCOL:
> +		case BPF_S_ANC_PKTTYPE:
> +		case BPF_S_ANC_IFINDEX:
> +		case BPF_S_ANC_MARK:
> +		case BPF_S_ANC_QUEUE:
> +		case BPF_S_ANC_HATYPE:
> +		case BPF_S_ANC_RXHASH:
> +		case BPF_S_ANC_CPU:
> +		case BPF_S_ANC_NLATTR:
> +		case BPF_S_ANC_NLATTR_NEST:
> +			/* ignored */
> +			continue;
> +		default:
> +			WARN_RATELIMIT(1, "Unknown code:%u jt:%u tf:%u k:%u\n",
> +				       fentry->code, fentry->jt,
> +				       fentry->jf, fentry->k);
> +			return 0;
> +		}
> +	}
> +
> +	return 0;
> +}
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 481611f..77f2eda 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1783,6 +1783,10 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>  		case PR_SET_SECCOMP:
>  			error = prctl_set_seccomp(arg2);
>  			break;
> +		case PR_ATTACH_SECCOMP_FILTER:
> +			error = prctl_attach_seccomp_filter((char __user *)
> +								arg2);
> +			break;
>  		case PR_GET_TSC:
>  			error = GET_TSC_CTL(arg2);
>  			break;
> diff --git a/security/Kconfig b/security/Kconfig
> index 51bd5a0..77b1106 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -84,6 +84,18 @@ 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 EXPERIMENTAL
> +	help
> +	  This kernel feature expands CONFIG_SECCOMP to allow computing
> +	  in environments with reduced kernel access dictated by a system
> +	  call filter, expressed in BPF, installed by the application itself
> +	  through prctl(2).
> +
> +	  See Documentation/prctl/seccomp_filter.txt for more detail.
> +
>  config SECURITY
>  	bool "Enable different security models"
>  	depends on SYSFS
> -- 
> 1.7.5.4
> 
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