1SECCOMP(2) Linux Programmer's Manual SECCOMP(2)
2
6 seccomp - operate on Secure Computing state of the process
7
9 #include <linux/seccomp.h>
10 #include <linux/filter.h>
11 #include <linux/audit.h>
12 #include <linux/signal.h>
13 #include <sys/ptrace.h>
14 int seccomp(unsigned int operation, unsigned int flags, void *args);
16
18 The seccomp() system call operates on the Secure Computing (seccomp)
19 state of the calling process.
20 Currently, Linux supports the following operation values:
22 SECCOMP_SET_MODE_STRICT
24 The only system calls that the calling thread is permitted to
25 make are read(2), write(2), _exit(2) (but not exit_group(2)),
26 and sigreturn(2). Other system calls result in the delivery of
27 a SIGKILL signal. Strict secure computing mode is useful for
28 number-crunching applications that may need to execute untrusted
29 byte code, perhaps obtained by reading from a pipe or socket.
30 Note that although the calling thread can no longer call sig‐
32 procmask(2), it can use sigreturn(2) to block all signals apart
33 from SIGKILL and SIGSTOP. This means that alarm(2) (for exam‐
34 ple) is not sufficient for restricting the process's execution
35 time. Instead, to reliably terminate the process, SIGKILL must
36 be used. This can be done by using timer_create(2) with
37 SIGEV_SIGNAL and sigev_signo set to SIGKILL, or by using setr‐
38 limit(2) to set the hard limit for RLIMIT_CPU.
39 This operation is available only if the kernel is configured
41 with CONFIG_SECCOMP enabled.
42 The value of flags must be 0, and args must be NULL.
44 This operation is functionally identical to the call:
46 prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);
48 SECCOMP_SET_MODE_FILTER
50 The system calls allowed are defined by a pointer to a Berkeley
51 Packet Filter (BPF) passed via args. This argument is a pointer
52 to a struct sock_fprog; it can be designed to filter arbitrary
53 system calls and system call arguments. If the filter is
54 invalid, seccomp() fails, returning EINVAL in errno.
55 If fork(2) or clone(2) is allowed by the filter, any child pro‐
57 cesses will be constrained to the same system call filters as
58 the parent. If execve(2) is allowed, the existing filters will
59 be preserved across a call to execve(2).
60 In order to use the SECCOMP_SET_MODE_FILTER operation, either
62 the calling thread must have the CAP_SYS_ADMIN capability in its
63 user namespace, or the thread must already have the no_new_privs
64 bit set. If that bit was not already set by an ancestor of this
65 thread, the thread must make the following call:
66 prctl(PR_SET_NO_NEW_PRIVS, 1);
68 Otherwise, the SECCOMP_SET_MODE_FILTER operation fails and
70 returns EACCES in errno. This requirement ensures that an
71 unprivileged process cannot apply a malicious filter and then
72 invoke a set-user-ID or other privileged program using
73 execve(2), thus potentially compromising that program. (Such a
74 malicious filter might, for example, cause an attempt to use
75 setuid(2) to set the caller's user IDs to nonzero values to
76 instead return 0 without actually making the system call. Thus,
77 the program might be tricked into retaining superuser privileges
78 in circumstances where it is possible to influence it to do dan‐
79 gerous things because it did not actually drop privileges.)
80 If prctl(2) or seccomp() is allowed by the attached filter, fur‐
82 ther filters may be added. This will increase evaluation time,
83 but allows for further reduction of the attack surface during
84 execution of a thread.
85 The SECCOMP_SET_MODE_FILTER operation is available only if the
87 kernel is configured with CONFIG_SECCOMP_FILTER enabled.
88 When flags is 0, this operation is functionally identical to the
90 call:
91 prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, args);
93 The recognized flags are:
95 SECCOMP_FILTER_FLAG_TSYNC
97 When adding a new filter, synchronize all other threads
98 of the calling process to the same seccomp filter tree.
99 A "filter tree" is the ordered list of filters attached
100 to a thread. (Attaching identical filters in separate
101 seccomp() calls results in different filters from this
102 perspective.)
103 If any thread cannot synchronize to the same filter tree,
105 the call will not attach the new seccomp filter, and will
106 fail, returning the first thread ID found that cannot
107 synchronize. Synchronization will fail if another thread
108 in the same process is in SECCOMP_MODE_STRICT or if it
109 has attached new seccomp filters to itself, diverging
110 from the calling thread's filter tree.
111 SECCOMP_FILTER_FLAG_LOG (since Linux 4.14)
113 All filter return actions except SECCOMP_RET_ALLOW should
114 be logged. An administrator may override this filter
115 flag by preventing specific actions from being logged via
116 the /proc/sys/kernel/seccomp/actions_logged file.
117 SECCOMP_FILTER_FLAG_SPEC_ALLOW (since Linux 4.17)
119 Disable Speculative Store Bypass mitigation.
120 SECCOMP_GET_ACTION_AVAIL (since Linux 4.14)
122 Test to see if an action is supported by the kernel. This oper‐
123 ation is helpful to confirm that the kernel knows of a more
124 recently added filter return action since the kernel treats all
125 unknown actions as SECCOMP_RET_KILL_PROCESS.
126 The value of flags must be 0, and args must be a pointer to an
128 unsigned 32-bit filter return action.
129 Filters
131 When adding filters via SECCOMP_SET_MODE_FILTER, args points to a fil‐
132 ter program:
133 struct sock_fprog {
135 unsigned short len; /* Number of BPF instructions */
136 struct sock_filter *filter; /* Pointer to array of
137 BPF instructions */
138 };
139 Each program must contain one or more BPF instructions:
141 struct sock_filter { /* Filter block */
143 __u16 code; /* Actual filter code */
144 __u8 jt; /* Jump true */
145 __u8 jf; /* Jump false */
146 __u32 k; /* Generic multiuse field */
147 };
148 When executing the instructions, the BPF program operates on the system
150 call information made available (i.e., use the BPF_ABS addressing mode)
151 as a (read-only) buffer of the following form:
152 struct seccomp_data {
154 int nr; /* System call number */
155 __u32 arch; /* AUDIT_ARCH_* value
156 (see <linux/audit.h>) */
157 __u64 instruction_pointer; /* CPU instruction pointer */
158 __u64 args[6]; /* Up to 6 system call arguments */
159 };
160 Because numbering of system calls varies between architectures and some
162 architectures (e.g., x86-64) allow user-space code to use the calling
163 conventions of multiple architectures (and the convention being used
164 may vary over the life of a process that uses execve(2) to execute
165 binaries that employ the different conventions), it is usually neces‐
166 sary to verify the value of the arch field.
167 It is strongly recommended to use an allow-list approach whenever pos‐
169 sible because such an approach is more robust and simple. A deny-list
170 will have to be updated whenever a potentially dangerous system call is
171 added (or a dangerous flag or option if those are deny-listed), and it
172 is often possible to alter the representation of a value without alter‐
173 ing its meaning, leading to a deny-list bypass. See also Caveats
174 below.
175 The arch field is not unique for all calling conventions. The x86-64
177 ABI and the x32 ABI both use AUDIT_ARCH_X86_64 as arch, and they run on
178 the same processors. Instead, the mask __X32_SYSCALL_BIT is used on
179 the system call number to tell the two ABIs apart.
180 This means that in order to create a seccomp-based deny-list for system
182 calls performed through the x86-64 ABI, it is necessary to not only
183 check that arch equals AUDIT_ARCH_X86_64, but also to explicitly reject
184 all system calls that contain __X32_SYSCALL_BIT in nr.
185 The instruction_pointer field provides the address of the machine-lan‐
187 guage instruction that performed the system call. This might be useful
188 in conjunction with the use of /proc/[pid]/maps to perform checks based
189 on which region (mapping) of the program made the system call. (Proba‐
190 bly, it is wise to lock down the mmap(2) and mprotect(2) system calls
191 to prevent the program from subverting such checks.)
192 When checking values from args against a deny-list, keep in mind that
194 arguments are often silently truncated before being processed, but
195 after the seccomp check. For example, this happens if the i386 ABI is
196 used on an x86-64 kernel: although the kernel will normally not look
197 beyond the 32 lowest bits of the arguments, the values of the full
198 64-bit registers will be present in the seccomp data. A less surpris‐
199 ing example is that if the x86-64 ABI is used to perform a system call
200 that takes an argument of type int, the more-significant half of the
201 argument register is ignored by the system call, but visible in the
202 seccomp data.
203 A seccomp filter returns a 32-bit value consisting of two parts: the
205 most significant 16 bits (corresponding to the mask defined by the con‐
206 stant SECCOMP_RET_ACTION_FULL) contain one of the "action" values
207 listed below; the least significant 16-bits (defined by the constant
208 SECCOMP_RET_DATA) are "data" to be associated with this return value.
209 If multiple filters exist, they are all executed, in reverse order of
211 their addition to the filter tree—that is, the most recently installed
212 filter is executed first. (Note that all filters will be called even
213 if one of the earlier filters returns SECCOMP_RET_KILL. This is done
214 to simplify the kernel code and to provide a tiny speed-up in the exe‐
215 cution of sets of filters by avoiding a check for this uncommon case.)
216 The return value for the evaluation of a given system call is the
217 first-seen action value of highest precedence (along with its accompa‐
218 nying data) returned by execution of all of the filters.
219 In decreasing order of precedence, the action values that may be
221 returned by a seccomp filter are:
222 SECCOMP_RET_KILL_PROCESS (since Linux 4.14)
224 This value results in immediate termination of the process, with
225 a core dump. The system call is not executed. By contrast with
226 SECCOMP_RET_KILL_THREAD below, all threads in the thread group
227 are terminated. (For a discussion of thread groups, see the
228 description of the CLONE_THREAD flag in clone(2).)
229 The process terminates as though killed by a SIGSYS signal.
231 Even if a signal handler has been registered for SIGSYS, the
232 handler will be ignored in this case and the process always ter‐
233 minates. To a parent process that is waiting on this process
234 (using waitpid(2) or similar), the returned wstatus will indi‐
235 cate that its child was terminated as though by a SIGSYS signal.
236 SECCOMP_RET_KILL_THREAD (or SECCOMP_RET_KILL)
238 This value results in immediate termination of the thread that
239 made the system call. The system call is not executed. Other
240 threads in the same thread group will continue to execute.
241 The thread terminates as though killed by a SIGSYS signal. See
243 SECCOMP_RET_KILL_PROCESS above.
244 Before Linux 4.11, any process terminated in this way would not
246 trigger a coredump (even though SIGSYS is documented in sig‐
247 nal(7) as having a default action of termination with a core
248 dump). Since Linux 4.11, a single-threaded process will dump
249 core if terminated in this way.
250 With the addition of SECCOMP_RET_KILL_PROCESS in Linux 4.14,
252 SECCOMP_RET_KILL_THREAD was added as a synonym for SEC‐
253 COMP_RET_KILL, in order to more clearly distinguish the two
254 actions.
255 SECCOMP_RET_TRAP
257 This value results in the kernel sending a thread-directed
258 SIGSYS signal to the triggering thread. (The system call is not
259 executed.) Various fields will be set in the siginfo_t struc‐
260 ture (see sigaction(2)) associated with signal:
261 * si_signo will contain SIGSYS.
263 * si_call_addr will show the address of the system call
265 instruction.
266 * si_syscall and si_arch will indicate which system call was
268 attempted.
269 * si_code will contain SYS_SECCOMP.
271 * si_errno will contain the SECCOMP_RET_DATA portion of the
273 filter return value.
274 The program counter will be as though the system call happened
276 (i.e., the program counter will not point to the system call
277 instruction). The return value register will contain an archi‐
278 tecture-dependent value; if resuming execution, set it to some‐
279 thing appropriate for the system call. (The architecture depen‐
280 dency is because replacing it with ENOSYS could overwrite some
281 useful information.)
282 SECCOMP_RET_ERRNO
284 This value results in the SECCOMP_RET_DATA portion of the fil‐
285 ter's return value being passed to user space as the errno value
286 without executing the system call.
287 SECCOMP_RET_TRACE
289 When returned, this value will cause the kernel to attempt to
290 notify a ptrace(2)-based tracer prior to executing the system
291 call. If there is no tracer present, the system call is not
292 executed and returns a failure status with errno set to ENOSYS.
293 A tracer will be notified if it requests PTRACE_O_TRACESECCOMP
295 using ptrace(PTRACE_SETOPTIONS). The tracer will be notified of
296 a PTRACE_EVENT_SECCOMP and the SECCOMP_RET_DATA portion of the
297 filter's return value will be available to the tracer via
298 PTRACE_GETEVENTMSG.
299 The tracer can skip the system call by changing the system call
301 number to -1. Alternatively, the tracer can change the system
302 call requested by changing the system call to a valid system
303 call number. If the tracer asks to skip the system call, then
304 the system call will appear to return the value that the tracer
305 puts in the return value register.
306 Before kernel 4.8, the seccomp check will not be run again after
308 the tracer is notified. (This means that, on older kernels,
309 seccomp-based sandboxes must not allow use of ptrace(2)—even of
310 other sandboxed processes—without extreme care; ptracers can use
311 this mechanism to escape from the seccomp sandbox.)
312 SECCOMP_RET_LOG (since Linux 4.14)
314 This value results in the system call being executed after the
315 filter return action is logged. An administrator may override
316 the logging of this action via the /proc/sys/kernel/sec‐
317 comp/actions_logged file.
318 SECCOMP_RET_ALLOW
320 This value results in the system call being executed.
321 If an action value other than one of the above is specified, then the
323 filter action is treated as either SECCOMP_RET_KILL_PROCESS (since
324 Linux 4.14) or SECCOMP_RET_KILL_THREAD (in Linux 4.13 and earlier).
325 /proc interfaces
327 The files in the directory /proc/sys/kernel/seccomp provide additional
328 seccomp information and configuration:
329 actions_avail (since Linux 4.14)
331 A read-only ordered list of seccomp filter return actions in
332 string form. The ordering, from left-to-right, is in decreasing
333 order of precedence. The list represents the set of seccomp
334 filter return actions supported by the kernel.
335 actions_logged (since Linux 4.14)
337 A read-write ordered list of seccomp filter return actions that
338 are allowed to be logged. Writes to the file do not need to be
339 in ordered form but reads from the file will be ordered in the
340 same way as the actions_avail file.
341 It is important to note that the value of actions_logged does
343 not prevent certain filter return actions from being logged when
344 the audit subsystem is configured to audit a task. If the
345 action is not found in the actions_logged file, the final deci‐
346 sion on whether to audit the action for that task is ultimately
347 left up to the audit subsystem to decide for all filter return
348 actions other than SECCOMP_RET_ALLOW.
349 The "allow" string is not accepted in the actions_logged file as
351 it is not possible to log SECCOMP_RET_ALLOW actions. Attempting
352 to write "allow" to the file will fail with the error EINVAL.
353 Audit logging of seccomp actions
355 Since Linux 4.14, the kernel provides the facility to log the actions
356 returned by seccomp filters in the audit log. The kernel makes the
357 decision to log an action based on the action type, whether or not the
358 action is present in the actions_logged file, and whether kernel audit‐
359 ing is enabled (e.g., via the kernel boot option audit=1). The rules
360 are as follows:
361 * If the action is SECCOMP_RET_ALLOW, the action is not logged.
363 * Otherwise, if the action is either SECCOMP_RET_KILL_PROCESS or SEC‐
365 COMP_RET_KILL_THREAD, and that action appears in the actions_logged
366 file, the action is logged.
367 * Otherwise, if the filter has requested logging (the SECCOMP_FIL‐
369 TER_FLAG_LOG flag) and the action appears in the actions_logged
370 file, the action is logged.
371 * Otherwise, if kernel auditing is enabled and the process is being
373 audited (autrace(8)), the action is logged.
374 * Otherwise, the action is not logged.
376
378 On success, seccomp() returns 0. On error, if SECCOMP_FIL‐
379 TER_FLAG_TSYNC was used, the return value is the ID of the thread that
380 caused the synchronization failure. (This ID is a kernel thread ID of
381 the type returned by clone(2) and gettid(2).) On other errors, -1 is
382 returned, and errno is set to indicate the cause of the error.
383
385 seccomp() can fail for the following reasons:
386 EACCES The caller did not have the CAP_SYS_ADMIN capability in its user
388 namespace, or had not set no_new_privs before using SEC‐
389 COMP_SET_MODE_FILTER.
390 EFAULT args was not a valid address.
392 EINVAL operation is unknown or is not supported by this kernel version
394 or configuration.
395 EINVAL The specified flags are invalid for the given operation.
397 EINVAL operation included BPF_ABS, but the specified offset was not
399 aligned to a 32-bit boundary or exceeded sizeof(struct sec‐
400 comp_data).
401 EINVAL A secure computing mode has already been set, and operation dif‐
403 fers from the existing setting.
404 EINVAL operation specified SECCOMP_SET_MODE_FILTER, but the filter pro‐
406 gram pointed to by args was not valid or the length of the fil‐
407 ter program was zero or exceeded BPF_MAXINSNS (4096) instruc‐
408 tions.
409 ENOMEM Out of memory.
411 ENOMEM The total length of all filter programs attached to the calling
413 thread would exceed MAX_INSNS_PER_PATH [22m(32768) instructions.
414 Note that for the purposes of calculating this limit, each
415 already existing filter program incurs an overhead penalty of 4
416 instructions.
417 EOPNOTSUPP
419 operation specified SECCOMP_GET_ACTION_AVAIL, but the kernel
420 does not support the filter return action specified by args.
421 ESRCH Another thread caused a failure during thread sync, but its ID
423 could not be determined.
424
426 The seccomp() system call first appeared in Linux 3.17.
427
429 The seccomp() system call is a nonstandard Linux extension.
430
432 Rather than hand-coding seccomp filters as shown in the example below,
433 you may prefer to employ the libseccomp library, which provides a
434 front-end for generating seccomp filters.
435 The Seccomp field of the /proc/[pid]/status file provides a method of
437 viewing the seccomp mode of a process; see proc(5).
438 seccomp() provides a superset of the functionality provided by the
440 prctl(2) PR_SET_SECCOMP operation (which does not support flags).
441 Since Linux 4.4, the ptrace(2) PTRACE_SECCOMP_GET_FILTER operation can
443 be used to dump a process's seccomp filters.
444 Architecture support for seccomp BPF
446 Architecture support for seccomp BPF filtering is available on the fol‐
447 lowing architectures:
448 * x86-64, i386, x32 (since Linux 3.5)
450 * ARM (since Linux 3.8)
451 * s390 (since Linux 3.8)
452 * MIPS (since Linux 3.16)
453 * ARM-64 (since Linux 3.19)
454 * PowerPC (since Linux 4.3)
455 * Tile (since Linux 4.3)
456 * PA-RISC (since Linux 4.6)
457 Caveats
459 There are various subtleties to consider when applying seccomp filters
460 to a program, including the following:
461 * Some traditional system calls have user-space implementations in the
463 vdso(7) on many architectures. Notable examples include clock_get‐
464 time(2), gettimeofday(2), and time(2). On such architectures, sec‐
465 comp filtering for these system calls will have no effect. (How‐
466 ever, there are cases where the vdso(7) implementations may fall
467 back to invoking the true system call, in which case seccomp filters
468 would see the system call.)
469 * Seccomp filtering is based on system call numbers. However, appli‐
471 cations typically do not directly invoke system calls, but instead
472 call wrapper functions in the C library which in turn invoke the
473 system calls. Consequently, one must be aware of the following:
474 · The glibc wrappers for some traditional system calls may actually
476 employ system calls with different names in the kernel. For
477 example, the exit(2) wrapper function actually employs the
478 exit_group(2) system call, and the fork(2) wrapper function actu‐
479 ally calls clone(2).
480 · The behavior of wrapper functions may vary across architectures,
482 according to the range of system calls provided on those archi‐
483 tectures. In other words, the same wrapper function may invoke
484 different system calls on different architectures.
485 · Finally, the behavior of wrapper functions can change across
487 glibc versions. For example, in older versions, the glibc wrap‐
488 per function for open(2) invoked the system call of the same
489 name, but starting in glibc 2.26, the implementation switched to
490 calling openat(2) on all architectures.
491 The consequence of the above points is that it may be necessary to fil‐
493 ter for a system call other than might be expected. Various manual
494 pages in Section 2 provide helpful details about the differences
495 between wrapper functions and the underlying system calls in subsec‐
496 tions entitled C library/kernel differences.
497 Furthermore, note that the application of seccomp filters even risks
499 causing bugs in an application, when the filters cause unexpected fail‐
500 ures for legitimate operations that the application might need to per‐
501 form. Such bugs may not easily be discovered when testing the seccomp
502 filters if the bugs occur in rarely used application code paths.
503 Seccomp-specific BPF details
505 Note the following BPF details specific to seccomp filters:
506 * The BPF_H and BPF_B size modifiers are not supported: all operations
508 must load and store (4-byte) words (BPF_W).
509 * To access the contents of the seccomp_data buffer, use the BPF_ABS
511 addressing mode modifier.
512 * The BPF_LEN addressing mode modifier yields an immediate mode oper‐
514 and whose value is the size of the seccomp_data buffer.
515
517 The program below accepts four or more arguments. The first three
518 arguments are a system call number, a numeric architecture identifier,
519 and an error number. The program uses these values to construct a BPF
520 filter that is used at run time to perform the following checks:
521 [1] If the program is not running on the specified architecture, the
523 BPF filter causes system calls to fail with the error ENOSYS.
524 [2] If the program attempts to execute the system call with the speci‐
526 fied number, the BPF filter causes the system call to fail, with
527 errno being set to the specified error number.
528 The remaining command-line arguments specify the pathname and addi‐
530 tional arguments of a program that the example program should attempt
531 to execute using execv(3) (a library function that employs the
532 execve(2) system call). Some example runs of the program are shown
533 below.
534 First, we display the architecture that we are running on (x86-64) and
536 then construct a shell function that looks up system call numbers on
537 this architecture:
538 $ uname -m
540 x86_64
541 $ syscall_nr() {
542 cat /usr/src/linux/arch/x86/syscalls/syscall_64.tbl | \
543 awk '$2 != "x32" && $3 == "'$1'" { print $1 }'
544 }
545 When the BPF filter rejects a system call (case [2] above), it causes
547 the system call to fail with the error number specified on the command
548 line. In the experiments shown here, we'll use error number 99:
549 $ errno 99
551 EADDRNOTAVAIL 99 Cannot assign requested address
552 In the following example, we attempt to run the command whoami(1), but
554 the BPF filter rejects the execve(2) system call, so that the command
555 is not even executed:
556 $ syscall_nr execve
558 59
559 $ ./a.out
560 Usage: ./a.out <syscall_nr> <arch> <errno> <prog> [<args>]
561 Hint for <arch>: AUDIT_ARCH_I386: 0x40000003
562 AUDIT_ARCH_X86_64: 0xC000003E
563 $ ./a.out 59 0xC000003E 99 /bin/whoami
564 execv: Cannot assign requested address
565 In the next example, the BPF filter rejects the write(2) system call,
567 so that, although it is successfully started, the whoami(1) command is
568 not able to write output:
569 $ syscall_nr write
571 1
572 $ ./a.out 1 0xC000003E 99 /bin/whoami
573 In the final example, the BPF filter rejects a system call that is not
575 used by the whoami(1) command, so it is able to successfully execute
576 and produce output:
577 $ syscall_nr preadv
579 295
580 $ ./a.out 295 0xC000003E 99 /bin/whoami
581 cecilia
582 Program source
584 #include <errno.h>
585 #include <stddef.h>
586 #include <stdio.h>
587 #include <stdlib.h>
588 #include <unistd.h>
589 #include <linux/audit.h>
590 #include <linux/filter.h>
591 #include <linux/seccomp.h>
592 #include <sys/prctl.h>
593 #define X32_SYSCALL_BIT 0x40000000
595 static int
597 install_filter(int syscall_nr, int t_arch, int f_errno)
598 {
599 unsigned int upper_nr_limit = 0xffffffff;
600 /* Assume that AUDIT_ARCH_X86_64 means the normal x86-64 ABI
602 (in the x32 ABI, all system calls have bit 30 set in the
603 'nr' field, meaning the numbers are >= X32_SYSCALL_BIT) */
604 if (t_arch == AUDIT_ARCH_X86_64)
605 upper_nr_limit = X32_SYSCALL_BIT - 1;
606 struct sock_filter filter[] = {
608 /* [0] Load architecture from 'seccomp_data' buffer into
609 accumulator */
610 BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
611 (offsetof(struct seccomp_data, arch))),
612 /* [1] Jump forward 5 instructions if architecture does not
614 match 't_arch' */
615 BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, t_arch, 0, 5),
616 /* [2] Load system call number from 'seccomp_data' buffer into
618 accumulator */
619 BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
620 (offsetof(struct seccomp_data, nr))),
621 /* [3] Check ABI - only needed for x86-64 in deny-list use
623 cases. Use BPF_JGT instead of checking against the bit
624 mask to avoid having to reload the syscall number. */
625 BPF_JUMP(BPF_JMP | BPF_JGT | BPF_K, upper_nr_limit, 3, 0),
626 /* [4] Jump forward 1 instruction if system call number
628 does not match 'syscall_nr' */
629 BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, syscall_nr, 0, 1),
630 /* [5] Matching architecture and system call: don't execute
632 the system call, and return 'f_errno' in 'errno' */
633 BPF_STMT(BPF_RET | BPF_K,
634 SECCOMP_RET_ERRNO | (f_errno & SECCOMP_RET_DATA)),
635 /* [6] Destination of system call number mismatch: allow other
637 system calls */
638 BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),
639 /* [7] Destination of architecture mismatch: kill task */
641 BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL),
642 };
643 struct sock_fprog prog = {
645 .len = (unsigned short) (sizeof(filter) / sizeof(filter[0])),
646 .filter = filter,
647 };
648 if (seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog)) {
650 perror("seccomp");
651 return 1;
652 }
653 return 0;
655 }
656 int
658 main(int argc, char **argv)
659 {
660 if (argc < 5) {
661 fprintf(stderr, "Usage: "
662 "%s <syscall_nr> <arch> <errno> <prog> [<args>]\n"
663 "Hint for <arch>: AUDIT_ARCH_I386: 0x%X\n"
664 " AUDIT_ARCH_X86_64: 0x%X\n"
665 "\n", argv[0], AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
666 exit(EXIT_FAILURE);
667 }
668 if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
670 perror("prctl");
671 exit(EXIT_FAILURE);
672 }
673 if (install_filter(strtol(argv[1], NULL, 0),
675 strtol(argv[2], NULL, 0),
676 strtol(argv[3], NULL, 0)))
677 exit(EXIT_FAILURE);
678 execv(argv[4], &argv[4]);
680 perror("execv");
681 exit(EXIT_FAILURE);
682 }
683
685 bpfc(1), strace(1), bpf(2), prctl(2), ptrace(2), sigaction(2), proc(5),
686 signal(7), socket(7)
687 Various pages from the libseccomp library, including:
689 scmp_sys_resolver(1), seccomp_init(3), seccomp_load(3), sec‐
690 comp_rule_add(3), and seccomp_export_bpf(3).
691 The kernel source files Documentation/networking/filter.txt and Docu‐
693 mentation/userspace-api/seccomp_filter.rst (or Documentation/prctl/sec‐
694 comp_filter.txt before Linux 4.13).
695 McCanne, S. and Jacobson, V. (1992) The BSD Packet Filter: A New Archi‐
697 tecture for User-level Packet Capture, Proceedings of the USENIX Winter
698 1993 Conference ⟨http://www.tcpdump.org/papers/bpf-usenix93.pdf⟩
699
701 This page is part of release 5.04 of the Linux man-pages project. A
702 description of the project, information about reporting bugs, and the
703 latest version of this page, can be found at
704 https://www.kernel.org/doc/man-pages/.
705Linux 2019-11-19 SECCOMP(2)