seccomp: add system call filtering using BPF

[This patch depends on [e-mail]'s no_new_privs patch:
https://lkml.org/lkml/2012/1/30/264
The whole series including Andrew's patches can be found here:
https://github.com/redpig/linux/tree/seccomp
Complete diff here:
https://github.com/redpig/linux/compare/1dc65fed...seccomp
]

This patch adds support for seccomp mode 2. Mode 2 introduces the
ability for unprivileged processes to install system call filtering
policy expressed in terms of a Berkeley Packet Filter (BPF) program.
This program will be evaluated in the kernel for each system call
the task makes and computes a result based on data in the format
of struct seccomp_data.

A filter program may be installed by calling:
struct sock_fprog fprog = { ... };
...
prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &fprog);

The return value of the filter program determines if the system call is
allowed to proceed or denied. If the first filter program installed
allows prctl(2) calls, then the above call may be made repeatedly
by a task to further reduce its access to the kernel. All attached
programs must be evaluated before a system call will be allowed to
proceed.

Filter programs will be inherited across fork/clone and execve.
However, if the task attaching the filter is unprivileged
(!CAP_SYS_ADMIN) the no_new_privs bit will be set on the task. This
ensures that unprivileged tasks cannot attach filters that affect
privileged tasks (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
- BPF optimization (and JIT'ing) are well understood
- Userland already knows its ABI: system call numbers and desired
arguments
- No time-of-check-time-of-use vulnerable data accesses are possible.
- system call arguments are loaded on access only to minimize copying
required for system call policy decisions.

Mode 2 support is restricted to architectures that enable
HAVE_ARCH_SECCOMP_FILTER. In this patch, the primary dependency is on
syscall_get_arguments(). The full desired scope of this feature will
add a few minor additional requirements expressed later in this series.
Based on discussion, SECCOMP_RET_ERRNO and SECCOMP_RET_TRACE seem to be
the desired additional functionality.

No architectures are enabled in this patch.

Signed-off-by: Will Drewry <[e-mail]>
Acked-by: Serge Hallyn <[e-mail]>
Reviewed-by: Indan Zupancic <[e-mail]>
Acked-by: Eric Paris <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>

v18: - rebase to v3.4-rc2
- s/chk/check/ ([e-mail],[e-mail])
- allocate with GFP_KERNEL|__GFP_NOWARN ([e-mail])
- add a comment for get_u32 regarding endianness (akpm@)
- fix other typos, style mistakes (akpm@)
- added acked-by
v17: - properly guard seccomp filter needed headers ([e-mail])
- tighten return mask to 0x7fff0000
v16: - no change
v15: - add a 4 instr penalty when counting a path to account for seccomp_filter
size ([e-mail])
- drop the max insns to 256KB ([e-mail])
- return ENOMEM if the max insns limit has been hit ([e-mail])
- move IP checks after args ([e-mail])
- drop !user_filter check ([e-mail])
- only allow explicit bpf codes ([e-mail])
- exit_code -> exit_sig
v14: - put/get_seccomp_filter takes struct task_struct
([e-mail],[e-mail])
- adds seccomp_chk_filter and drops general bpf_run/chk_filter user
- add seccomp_bpf_load for use by net/core/filter.c
- lower max per-process/per-hierarchy: 1MB
- moved nnp/capability check prior to allocation
(all of the above: [e-mail])
v13: - rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc
v12: - added a maximum instruction count per path ([e-mail],[e-mail])
- removed copy_seccomp ([e-mail],[e-mail])
- reworded the prctl_set_seccomp comment ([e-mail])
v11: - reorder struct seccomp_data to allow future args expansion ([e-mail])
- style clean up, @compat dropped, compat_sock_fprog32 ([e-mail])
- do_exit(SIGSYS) ([e-mail], [e-mail])
- pare down Kconfig doc reference.
- extra comment clean up
v10: - seccomp_data has changed again to be more aesthetically pleasing
([e-mail])
- calling convention is noted in a new u32 field using syscall_get_arch.
This allows for cross-calling convention tasks to use seccomp filters.
([e-mail])
- lots of clean up (thanks, Indan!)
v9: - n/a
v8: - use bpf_chk_filter, bpf_run_filter. update load_fns
- Lots of fixes courtesy of [e-mail]:
-- fix up load behavior, compat fixups, and merge alloc code,
-- renamed pc and dropped __packed, use bool compat.
-- Added a hidden CONFIG_SECCOMP_FILTER to synthesize non-arch
dependencies
v7: (massive overhaul thanks to Indan, others)
- added CONFIG_HAVE_ARCH_SECCOMP_FILTER
- merged into seccomp.c
- minimal seccomp_filter.h
- no config option (part of seccomp)
- no new prctl
- doesn't break seccomp on systems without asm/syscall.h
(works but arg access always fails)
- dropped seccomp_init_task, extra free functions, ...
- dropped the no-asm/syscall.h code paths
- merges with network sk_run_filter and sk_chk_filter
v6: - fix memory leak on attach compat check failure
- require no_new_privs || CAP_SYS_ADMIN prior to filter
installation. ([e-mail])
- s/seccomp_struct_/seccomp_/ for macros/functions ([e-mail])
- cleaned up Kconfig ([e-mail])
- on block, note if the call was compat (so the # means something)
v5: - uses syscall_get_arguments
([e-mail],[e-mail], [e-mail])
- uses union-based arg storage with hi/lo struct to
handle endianness. Compromises between the two alternate
proposals to minimize extra arg shuffling and account for
endianness assuming userspace uses offsetof().
([e-mail], [e-mail])
- update Kconfig description
- add include/seccomp_filter.h and add its installation
- (naive) on-demand syscall argument loading
- drop seccomp_t ([e-mail])
v4: - adjusted prctl to make room for PR_[SG]ET_NO_NEW_PRIVS
- now uses current->no_new_privs
([e-mail],[e-mail])
- assign names to seccomp modes ([e-mail])
- fix style issues ([e-mail])
- reworded Kconfig entry ([e-mail])
v3: - macros to inline ([e-mail])
- init_task behavior fixed ([e-mail])
- drop creator entry and extra NULL check ([e-mail])
- alloc returns -EINVAL on bad sizing ([e-mail])
- adds tentative use of "always_unprivileged" as per
[e-mail] and [e-mail]
v2: - (patch 2 only)
Signed-off-by: James Morris <[e-mail]>

arch/Kconfig

···
 config ARCH_WANT_OLD_COMPAT_IPC
 	bool
 
+config HAVE_ARCH_SECCOMP_FILTER
+	bool
+	help
+	  This symbol should be selected by an architecure if it provides
+	  asm/syscall.h, specifically syscall_get_arguments() and
+	  syscall_get_arch().
+
+config SECCOMP_FILTER
+	def_bool y
+	depends on HAVE_ARCH_SECCOMP_FILTER && SECCOMP && NET
+	help
+	  Enable tasks to build secure computing environments defined
+	  in terms of Berkeley Packet Filter programs which implement
+	  task-defined system call filtering polices.
+
+	  See Documentation/prctl/seccomp_filter.txt for details.
+
 source "kernel/gcov/Kconfig"

include/linux/Kbuild

···
 header-y += sched.h
 header-y += screen_info.h
 header-y += sdla.h
+header-y += seccomp.h
 header-y += securebits.h
 header-y += selinux_netlink.h
 header-y += sem.h

include/linux/seccomp.h

···
 #ifndef _LINUX_SECCOMP_H
 #define _LINUX_SECCOMP_H
 
+#include <linux/compiler.h>
+#include <linux/types.h>
 
+
+/* Valid values for seccomp.mode and prctl(PR_SET_SECCOMP, <mode>) */
+#define SECCOMP_MODE_DISABLED	0 /* seccomp is not in use. */
+#define SECCOMP_MODE_STRICT	1 /* uses hard-coded filter. */
+#define SECCOMP_MODE_FILTER	2 /* uses user-supplied filter. */
+
+/*
+ * All BPF programs must return a 32-bit value.
+ * The bottom 16-bits are reserved for future use.
+ * The upper 16-bits are ordered from least permissive values to most.
+ *
+ * The ordering ensures that a min_t() over composed return values always
+ * selects the least permissive choice.
+ */
+#define SECCOMP_RET_KILL	0x00000000U /* kill the task immediately */
+#define SECCOMP_RET_ALLOW	0x7fff0000U /* allow */
+
+/* Masks for the return value sections. */
+#define SECCOMP_RET_ACTION	0x7fff0000U
+#define SECCOMP_RET_DATA	0x0000ffffU
+
+/**
+ * struct seccomp_data - the format the BPF program executes over.
+ * @nr: the system call number
+ * @arch: indicates system call convention as an AUDIT_ARCH_* value
+ *        as defined in <linux/audit.h>.
+ * @instruction_pointer: at the time of the system call.
+ * @args: up to 6 system call arguments always stored as 64-bit values
+ *        regardless of the architecture.
+ */
+struct seccomp_data {
+	int nr;
+	__u32 arch;
+	__u64 instruction_pointer;
+	__u64 args[6];
+};
+
+#ifdef __KERNEL__
 #ifdef CONFIG_SECCOMP
 
 #include <linux/thread_info.h>
 #include <asm/seccomp.h>
 
+struct seccomp_filter;
+/**
+ * struct seccomp - the state of a seccomp'ed process
+ *
+ * @mode:  indicates one of the valid values above for controlled
+ *         system calls available to a process.
+ * @filter: The metadata and ruleset for determining what system calls
+ *          are allowed for a task.
+ *
+ *          @filter must only be accessed from the context of current as there
+ *          is no locking.
+ */
 struct seccomp {
 	int mode;
+	struct seccomp_filter *filter;
 };
 
 extern void __secure_computing(int);
···
 }
 
 extern long prctl_get_seccomp(void);
-extern long prctl_set_seccomp(unsigned long);
+extern long prctl_set_seccomp(unsigned long, char __user *);
 
 static inline int seccomp_mode(struct seccomp *s)
 {
···
 #include <linux/errno.h>
 
 struct seccomp { };
+struct seccomp_filter { };
 
-#define secure_computing(x) do { } while (0)
+#define secure_computing(x) 0
 
 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 arg2, char __user *arg3)
 {
 	return -EINVAL;
 }
···
 {
 	return 0;
 }
-
 #endif /* CONFIG_SECCOMP */
 
+#ifdef CONFIG_SECCOMP_FILTER
+extern void put_seccomp_filter(struct task_struct *tsk);
+extern void get_seccomp_filter(struct task_struct *tsk);
+extern u32 seccomp_bpf_load(int off);
+#else  /* CONFIG_SECCOMP_FILTER */
+static inline void put_seccomp_filter(struct task_struct *tsk)
+{
+	return;
+}
+static inline void get_seccomp_filter(struct task_struct *tsk)
+{
+	return;
+}
+#endif /* CONFIG_SECCOMP_FILTER */
+#endif /* __KERNEL__ */
 #endif /* _LINUX_SECCOMP_H */

kernel/fork.c

···
 #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>
···
 	free_thread_info(tsk->stack);
 	rt_mutex_debug_task_free(tsk);
 	ftrace_graph_exit_task(tsk);
+	put_seccomp_filter(tsk);
 	free_task_struct(tsk);
 }
 EXPORT_SYMBOL(free_task);
···
 		goto fork_out;
 
 	ftrace_graph_init_task(p);
+	get_seccomp_filter(p);
 
 	rt_mutex_init_task(p);
 

kernel/seccomp.c

···
  *
  * Copyright 2004-2005  Andrea Arcangeli <[e-mail]>
  *
- * This defines a simple but solid secure-computing mode.
+ * Copyright (C) 2012 Google, Inc.
+ * Will Drewry <[e-mail]>
+ *
+ * This defines a simple but solid secure-computing facility.
+ *
+ * Mode 1 uses a fixed list of allowed system calls.
+ * Mode 2 allows user-defined system call filters in the form
+ *        of Berkeley Packet Filters/Linux Socket Filters.
  */
 
+#include <linux/atomic.h>
 #include <linux/audit.h>
-#include <linux/seccomp.h>
-#include <linux/sched.h>
 #include <linux/compat.h>
+#include <linux/sched.h>
+#include <linux/seccomp.h>
 
 /* #define SECCOMP_DEBUG 1 */
-#define NR_SECCOMP_MODES 1
+
+#ifdef CONFIG_SECCOMP_FILTER
+#include <asm/syscall.h>
+#include <linux/filter.h>
+#include <linux/security.h>
+#include <linux/slab.h>
+#include <linux/tracehook.h>
+#include <linux/uaccess.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.
+ * @prev: points to a previously installed, or inherited, filter
+ * @len: the number of instructions in the program
+ * @insns: the BPF program instructions to evaluate
+ *
+ * seccomp_filter objects are organized in a tree linked via the @prev
+ * pointer.  For any task, it appears to be a singly-linked list starting
+ * with current->seccomp.filter, the most recently attached or inherited filter.
+ * However, multiple filters may share a @prev node, by way of fork(), which
+ * results in a unidirectional tree existing in memory.  This is similar to
+ * how namespaces work.
+ *
+ * seccomp_filter objects should never be modified after being attached
+ * to a task_struct (other than @usage).
+ */
+struct seccomp_filter {
+	atomic_t usage;
+	struct seccomp_filter *prev;
+	unsigned short len;  /* Instruction count */
+	struct sock_filter insns[];
+};
+
+/* Limit any path through the tree to 256KB worth of instructions. */
+#define MAX_INSNS_PER_PATH ((1 << 18) / sizeof(struct sock_filter))
+
+static void seccomp_filter_log_failure(int syscall)
+{
+	int compat = 0;
+#ifdef CONFIG_COMPAT
+	compat = is_compat_task();
+#endif
+	pr_info("%s[%d]: %ssystem call %d blocked at 0x%lx\n",
+		current->comm, task_pid_nr(current),
+		(compat ? "compat " : ""),
+		syscall, KSTK_EIP(current));
+}
+
+/**
+ * get_u32 - returns a u32 offset into data
+ * @data: a unsigned 64 bit value
+ * @index: 0 or 1 to return the first or second 32-bits
+ *
+ * This inline exists to hide the length of unsigned long.  If a 32-bit
+ * unsigned long is passed in, it will be extended and the top 32-bits will be
+ * 0. If it is a 64-bit unsigned long, then whatever data is resident will be
+ * properly returned.
+ *
+ * Endianness is explicitly ignored and left for BPF program authors to manage
+ * as per the specific architecture.
+ */
+static inline u32 get_u32(u64 data, int index)
+{
+	return ((u32 *)&data)[index];
+}
+
+/* Helper for bpf_load below. */
+#define BPF_DATA(_name) offsetof(struct seccomp_data, _name)
+/**
+ * bpf_load: checks and returns a pointer to the requested offset
+ * @off: offset into struct seccomp_data to load from
+ *
+ * Returns the requested 32-bits of data.
+ * seccomp_check_filter() should assure that @off is 32-bit aligned
+ * and not out of bounds.  Failure to do so is a BUG.
+ */
+u32 seccomp_bpf_load(int off)
+{
+	struct pt_regs *regs = task_pt_regs(current);
+	if (off == BPF_DATA(nr))
+		return syscall_get_nr(current, regs);
+	if (off == BPF_DATA(arch))
+		return syscall_get_arch(current, regs);
+	if (off >= BPF_DATA(args[0]) && off < BPF_DATA(args[6])) {
+		unsigned long value;
+		int arg = (off - BPF_DATA(args[0])) / sizeof(u64);
+		int index = !!(off % sizeof(u64));
+		syscall_get_arguments(current, regs, arg, 1, &value);
+		return get_u32(value, index);
+	}
+	if (off == BPF_DATA(instruction_pointer))
+		return get_u32(KSTK_EIP(current), 0);
+	if (off == BPF_DATA(instruction_pointer) + sizeof(u32))
+		return get_u32(KSTK_EIP(current), 1);
+	/* seccomp_check_filter should make this impossible. */
+	BUG();
+}
+
+/**
+ *	seccomp_check_filter - verify seccomp filter code
+ *	@filter: filter to verify
+ *	@flen: length of filter
+ *
+ * Takes a previously checked filter (by sk_chk_filter) and
+ * redirects all filter code that loads struct sk_buff data
+ * and related data through seccomp_bpf_load.  It also
+ * enforces length and alignment checking of those loads.
+ *
+ * Returns 0 if the rule set is legal or -EINVAL if not.
+ */
+static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
+{
+	int pc;
+	for (pc = 0; pc < flen; pc++) {
+		struct sock_filter *ftest = &filter[pc];
+		u16 code = ftest->code;
+		u32 k = ftest->k;
+
+		switch (code) {
+		case BPF_S_LD_W_ABS:
+			ftest->code = BPF_S_ANC_SECCOMP_LD_W;
+			/* 32-bit aligned and not out of bounds. */
+			if (k >= sizeof(struct seccomp_data) || k & 3)
+				return -EINVAL;
+			continue;
+		case BPF_S_LD_W_LEN:
+			ftest->code = BPF_S_LD_IMM;
+			ftest->k = sizeof(struct seccomp_data);
+			continue;
+		case BPF_S_LDX_W_LEN:
+			ftest->code = BPF_S_LDX_IMM;
+			ftest->k = sizeof(struct seccomp_data);
+			continue;
+		/* Explicitly include allowed calls. */
+		case BPF_S_RET_K:
+		case BPF_S_RET_A:
+		case BPF_S_ALU_ADD_K:
+		case BPF_S_ALU_ADD_X:
+		case BPF_S_ALU_SUB_K:
+		case BPF_S_ALU_SUB_X:
+		case BPF_S_ALU_MUL_K:
+		case BPF_S_ALU_MUL_X:
+		case BPF_S_ALU_DIV_X:
+		case BPF_S_ALU_AND_K:
+		case BPF_S_ALU_AND_X:
+		case BPF_S_ALU_OR_K:
+		case BPF_S_ALU_OR_X:
+		case BPF_S_ALU_LSH_K:
+		case BPF_S_ALU_LSH_X:
+		case BPF_S_ALU_RSH_K:
+		case BPF_S_ALU_RSH_X:
+		case BPF_S_ALU_NEG:
+		case BPF_S_LD_IMM:
+		case BPF_S_LDX_IMM:
+		case BPF_S_MISC_TAX:
+		case BPF_S_MISC_TXA:
+		case BPF_S_ALU_DIV_K:
+		case BPF_S_LD_MEM:
+		case BPF_S_LDX_MEM:
+		case BPF_S_ST:
+		case BPF_S_STX:
+		case BPF_S_JMP_JA:
+		case BPF_S_JMP_JEQ_K:
+		case BPF_S_JMP_JEQ_X:
+		case BPF_S_JMP_JGE_K:
+		case BPF_S_JMP_JGE_X:
+		case BPF_S_JMP_JGT_K:
+		case BPF_S_JMP_JGT_X:
+		case BPF_S_JMP_JSET_K:
+		case BPF_S_JMP_JSET_X:
+			continue;
+		default:
+			return -EINVAL;
+		}
+	}
+	return 0;
+}
+
+/**
+ * seccomp_run_filters - evaluates all seccomp filters against @syscall
+ * @syscall: number of the current system call
+ *
+ * Returns valid seccomp BPF response codes.
+ */
+static u32 seccomp_run_filters(int syscall)
+{
+	struct seccomp_filter *f;
+	u32 ret = SECCOMP_RET_KILL;
+	/*
+	 * All filters in the list are evaluated and the lowest BPF return
+	 * value always takes priority.
+	 */
+	for (f = current->seccomp.filter; f; f = f->prev) {
+		ret = sk_run_filter(NULL, f->insns);
+		if (ret != SECCOMP_RET_ALLOW)
+			break;
+	}
+	return ret;
+}
+
+/**
+ * seccomp_attach_filter: Attaches a seccomp filter to current.
+ * @fprog: BPF program to install
+ *
+ * Returns 0 on success or an errno on failure.
+ */
+static long seccomp_attach_filter(struct sock_fprog *fprog)
+{
+	struct seccomp_filter *filter;
+	unsigned long fp_size = fprog->len * sizeof(struct sock_filter);
+	unsigned long total_insns = fprog->len;
+	long ret;
+
+	if (fprog->len == 0 || fprog->len > BPF_MAXINSNS)
+		return -EINVAL;
+
+	for (filter = current->seccomp.filter; filter; filter = filter->prev)
+		total_insns += filter->len + 4;  /* include a 4 instr penalty */
+	if (total_insns > MAX_INSNS_PER_PATH)
+		return -ENOMEM;
+
+	/*
+	 * Installing a seccomp filter requires that the task have
+	 * CAP_SYS_ADMIN in its namespace or be running with no_new_privs.
+	 * This avoids scenarios where unprivileged tasks can affect the
+	 * behavior of privileged children.
+	 */
+	if (!current->no_new_privs &&
+	    security_capable_noaudit(current_cred(), current_user_ns(),
+				     CAP_SYS_ADMIN) != 0)
+		return -EACCES;
+
+	/* Allocate a new seccomp_filter */
+	filter = kzalloc(sizeof(struct seccomp_filter) + fp_size,
+			 GFP_KERNEL|__GFP_NOWARN);
+	if (!filter)
+		return -ENOMEM;
+	atomic_set(&filter->usage, 1);
+	filter->len = fprog->len;
+
+	/* Copy the instructions from fprog. */
+	ret = -EFAULT;
+	if (copy_from_user(filter->insns, fprog->filter, fp_size))
+		goto fail;
+
+	/* Check and rewrite the fprog via the skb checker */
+	ret = sk_chk_filter(filter->insns, filter->len);
+	if (ret)
+		goto fail;
+
+	/* Check and rewrite the fprog for seccomp use */
+	ret = seccomp_check_filter(filter->insns, filter->len);
+	if (ret)
+		goto fail;
+
+	/*
+	 * If there is an existing filter, make it the prev and don't drop its
+	 * task reference.
+	 */
+	filter->prev = current->seccomp.filter;
+	current->seccomp.filter = filter;
+	return 0;
+fail:
+	kfree(filter);
+	return ret;
+}
+
+/**
+ * seccomp_attach_user_filter - attaches a user-supplied sock_fprog
+ * @user_filter: pointer to the user data containing a sock_fprog.
+ *
+ * Returns 0 on success and non-zero otherwise.
+ */
+long seccomp_attach_user_filter(char __user *user_filter)
+{
+	struct sock_fprog fprog;
+	long ret = -EFAULT;
+
+#ifdef CONFIG_COMPAT
+	if (is_compat_task()) {
+		struct compat_sock_fprog fprog32;
+		if (copy_from_user(&fprog32, user_filter, sizeof(fprog32)))
+			goto out;
+		fprog.len = fprog32.len;
+		fprog.filter = compat_ptr(fprog32.filter);
+	} else /* falls through to the if below. */
+#endif
+	if (copy_from_user(&fprog, user_filter, sizeof(fprog)))
+		goto out;
+	ret = seccomp_attach_filter(&fprog);
+out:
+	return ret;
+}
+
+/* get_seccomp_filter - increments the reference count of the filter on @tsk */
+void get_seccomp_filter(struct task_struct *tsk)
+{
+	struct seccomp_filter *orig = tsk->seccomp.filter;
+	if (!orig)
+		return;
+	/* Reference count is bounded by the number of total processes. */
+	atomic_inc(&orig->usage);
+}
+
+/* put_seccomp_filter - decrements the ref count of tsk->seccomp.filter */
+void put_seccomp_filter(struct task_struct *tsk)
+{
+	struct seccomp_filter *orig = tsk->seccomp.filter;
+	/* Clean up single-reference branches iteratively. */
+	while (orig && atomic_dec_and_test(&orig->usage)) {
+		struct seccomp_filter *freeme = orig;
+		orig = orig->prev;
+		kfree(freeme);
+	}
+}
+#endif	/* CONFIG_SECCOMP_FILTER */
 
 /*
  * Secure computing mode 1 allows only read/write/exit/sigreturn.
···
 void __secure_computing(int this_syscall)
 {
 	int mode = current->seccomp.mode;
-	int * syscall;
+	int exit_sig = 0;
+	int *syscall;
 
 	switch (mode) {
-	case 1:
+	case SECCOMP_MODE_STRICT:
 		syscall = mode1_syscalls;
 #ifdef CONFIG_COMPAT
 		if (is_compat_task())
···
 			if (*syscall == this_syscall)
 				return;
 		} while (*++syscall);
+		exit_sig = SIGKILL;
 		break;
+#ifdef CONFIG_SECCOMP_FILTER
+	case SECCOMP_MODE_FILTER:
+		if (seccomp_run_filters(this_syscall) == SECCOMP_RET_ALLOW)
+			return;
+		seccomp_filter_log_failure(this_syscall);
+		exit_sig = SIGSYS;
+		break;
+#endif
 	default:
 		BUG();
 	}
···
 	dump_stack();
 #endif
 	audit_seccomp(this_syscall);
-	do_exit(SIGKILL);
+	do_exit(exit_sig);
 }
 
 long prctl_get_seccomp(void)
···
 	return current->seccomp.mode;
 }
 
-long prctl_set_seccomp(unsigned long seccomp_mode)
+/**
+ * prctl_set_seccomp: configures current->seccomp.mode
+ * @seccomp_mode: requested mode to use
+ * @filter: optional struct sock_fprog for use with SECCOMP_MODE_FILTER
+ *
+ * This function may be called repeatedly with a @seccomp_mode of
+ * SECCOMP_MODE_FILTER to install additional filters.  Every filter
+ * successfully installed will be evaluated (in reverse order) for each system
+ * call the task makes.
+ *
+ * Once current->seccomp.mode is non-zero, it may not be changed.
+ *
+ * Returns 0 on success or -EINVAL on failure.
+ */
+long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
 {
-	long ret;
+	long ret = -EINVAL;
 
-	/* can set it only once to be even more secure */
-	ret = -EPERM;
-	if (unlikely(current->seccomp.mode))
+	if (current->seccomp.mode &&
+	    current->seccomp.mode != seccomp_mode)
 		goto out;
 
-	ret = -EINVAL;
-	if (seccomp_mode && seccomp_mode <= NR_SECCOMP_MODES) {
-		current->seccomp.mode = seccomp_mode;
-		set_thread_flag(TIF_SECCOMP);
+	switch (seccomp_mode) {
+	case SECCOMP_MODE_STRICT:
+		ret = 0;
 #ifdef TIF_NOTSC
 		disable_TSC();
 #endif
-		ret = 0;
+		break;
+#ifdef CONFIG_SECCOMP_FILTER
+	case SECCOMP_MODE_FILTER:
+		ret = seccomp_attach_user_filter(filter);
+		if (ret)
+			goto out;
+		break;
+#endif
+	default:
+		goto out;
 	}
 
- out:
+	current->seccomp.mode = seccomp_mode;
+	set_thread_flag(TIF_SECCOMP);
+out:
 	return ret;
 }

kernel/sys.c

···
 			error = prctl_get_seccomp();
 			break;
 		case PR_SET_SECCOMP:
-			error = prctl_set_seccomp(arg2);
+			error = prctl_set_seccomp(arg2, (char __user *)arg3);
 			break;
 		case PR_GET_TSC:
 			error = GET_TSC_CTL(arg2);