x SuSE Linux 13.1-RELEASE x
x SuSE Linux 13.1-RELEASEx
seccomp(2) System Calls Manual seccomp(2)
NAME
seccomp - operate on Secure Computing state of the process
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <linux/seccomp.h> /* Definition of SECCOMP_* constants */
#include <linux/filter.h> /* Definition of struct sock_fprog */
#include <linux/audit.h> /* Definition of AUDIT_* constants */
#include <linux/signal.h> /* Definition of SIG* constants */
#include <sys/ptrace.h> /* Definition of PTRACE_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_seccomp, unsigned int operation, unsigned int flags,
void *args);
Note: glibc provides no wrapper for seccomp(), necessitating the use of
syscall(2).
DESCRIPTION
The seccomp() system call operates on the Secure Computing (seccomp)
state of the calling process.
Currently, Linux supports the following operation values:
SECCOMP_SET_MODE_STRICT
The only system calls that the calling thread is permitted to
make are read(2), write(2), _exit(2) (but not exit_group(2)),
and sigreturn(2). Other system calls result in the termination
of the calling thread, or termination of the entire process with
the SIGKILL signal when there is only one thread. Strict secure
computing mode is useful for number-crunching applications that
may need to execute untrusted byte code, perhaps obtained by
reading from a pipe or socket.
Note that although the calling thread can no longer call sig-
procmask(2), it can use sigreturn(2) to block all signals apart
from SIGKILL and SIGSTOP. This means that alarm(2) (for exam-
ple) is not sufficient for restricting the process's execution
time. Instead, to reliably terminate the process, SIGKILL must
be used. This can be done by using timer_create(2) with
SIGEV_SIGNAL and sigev_signo set to SIGKILL, or by using setr-
limit(2) to set the hard limit for RLIMIT_CPU.
This operation is available only if the kernel is configured
with CONFIG_SECCOMP enabled.
The value of flags must be 0, and args must be NULL.
This operation is functionally identical to the call:
prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);
SECCOMP_SET_MODE_FILTER
The system calls allowed are defined by a pointer to a Berkeley
Packet Filter (BPF) passed via args. This argument is a pointer
to a struct sock_fprog; it can be designed to filter arbitrary
system calls and system call arguments. If the filter is in-
valid, seccomp() fails, returning EINVAL in errno.
If fork(2) or clone(2) is allowed by the filter, any child pro-
cesses will be constrained to the same system call filters as
the parent. If execve(2) is allowed, the existing filters will
be preserved across a call to execve(2).
In order to use the SECCOMP_SET_MODE_FILTER operation, either
the calling thread must have the CAP_SYS_ADMIN capability in its
user namespace, or the thread must already have the no_new_privs
bit set. If that bit was not already set by an ancestor of this
thread, the thread must make the following call:
prctl(PR_SET_NO_NEW_PRIVS, 1);
Otherwise, the SECCOMP_SET_MODE_FILTER operation fails and re-
turns EACCES in errno. This requirement ensures that an unpriv-
ileged process cannot apply a malicious filter and then invoke a
set-user-ID or other privileged program using execve(2), thus
potentially compromising that program. (Such a malicious filter
might, for example, cause an attempt to use setuid(2) to set the
caller's user IDs to nonzero values to instead return 0 without
actually making the system call. Thus, the program might be
tricked into retaining superuser privileges in circumstances
where it is possible to influence it to do dangerous things be-
cause it did not actually drop privileges.)
If prctl(2) or seccomp() is allowed by the attached filter, fur-
ther filters may be added. This will increase evaluation time,
but allows for further reduction of the attack surface during
execution of a thread.
The SECCOMP_SET_MODE_FILTER operation is available only if the
kernel is configured with CONFIG_SECCOMP_FILTER enabled.
When flags is 0, this operation is functionally identical to the
call:
prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, args);
The recognized flags are:
SECCOMP_FILTER_FLAG_LOG (since Linux 4.14)
All filter return actions except SECCOMP_RET_ALLOW should
be logged. An administrator may override this filter
flag by preventing specific actions from being logged via
the /proc/sys/kernel/seccomp/actions_logged file.
SECCOMP_FILTER_FLAG_NEW_LISTENER (since Linux 5.0)
After successfully installing the filter program, return
a new user-space notification file descriptor. (The
close-on-exec flag is set for the file descriptor.) When
the filter returns SECCOMP_RET_USER_NOTIF a notification
will be sent to this file descriptor.
At most one seccomp filter using the SECCOMP_FIL-
TER_FLAG_NEW_LISTENER flag can be installed for a thread.
See seccomp_unotify(2) for further details.
SECCOMP_FILTER_FLAG_SPEC_ALLOW (since Linux 4.17)
Disable Speculative Store Bypass mitigation.
SECCOMP_FILTER_FLAG_TSYNC
When adding a new filter, synchronize all other threads
of the calling process to the same seccomp filter tree.
A "filter tree" is the ordered list of filters attached
to a thread. (Attaching identical filters in separate
seccomp() calls results in different filters from this
perspective.)
If any thread cannot synchronize to the same filter tree,
the call will not attach the new seccomp filter, and will
fail, returning the first thread ID found that cannot
synchronize. Synchronization will fail if another thread
in the same process is in SECCOMP_MODE_STRICT or if it
has attached new seccomp filters to itself, diverging
from the calling thread's filter tree.
SECCOMP_GET_ACTION_AVAIL (since Linux 4.14)
Test to see if an action is supported by the kernel. This oper-
ation is helpful to confirm that the kernel knows of a more re-
cently added filter return action since the kernel treats all
unknown actions as SECCOMP_RET_KILL_PROCESS.
The value of flags must be 0, and args must be a pointer to an
unsigned 32-bit filter return action.
SECCOMP_GET_NOTIF_SIZES (since Linux 5.0)
Get the sizes of the seccomp user-space notification structures.
Since these structures may evolve and grow over time, this com-
mand can be used to determine how much memory to allocate for
sending and receiving notifications.
The value of flags must be 0, and args must be a pointer to a
struct seccomp_notif_sizes, which has the following form:
struct seccomp_notif_sizes
__u16 seccomp_notif; /* Size of notification structure */
__u16 seccomp_notif_resp; /* Size of response structure */
__u16 seccomp_data; /* Size of 'struct seccomp_data' */
};
See seccomp_unotify(2) for further details.
Filters
When adding filters via SECCOMP_SET_MODE_FILTER, args points to a fil-
ter program:
struct sock_fprog {
unsigned short len; /* Number of BPF instructions */
struct sock_filter *filter; /* Pointer to array of
BPF instructions */
};
Each program must contain one or more BPF instructions:
struct sock_filter { /* Filter block */
__u16 code; /* Actual filter code */
__u8 jt; /* Jump true */
__u8 jf; /* Jump false */
__u32 k; /* Generic multiuse field */
};
When executing the instructions, the BPF program operates on the system
call information made available (i.e., use the BPF_ABS addressing mode)
as a (read-only) buffer of the following form:
struct seccomp_data {
int nr; /* System call number */
__u32 arch; /* AUDIT_ARCH_* value
(see <linux/audit.h>) */
__u64 instruction_pointer; /* CPU instruction pointer */
__u64 args[6]; /* Up to 6 system call arguments */
};
Because numbering of system calls varies between architectures and some
architectures (e.g., x86-64) allow user-space code to use the calling
conventions of multiple architectures (and the convention being used
may vary over the life of a process that uses execve(2) to execute bi-
naries that employ the different conventions), it is usually necessary
to verify the value of the arch field.
It is strongly recommended to use an allow-list approach whenever pos-
sible because such an approach is more robust and simple. A deny-list
will have to be updated whenever a potentially dangerous system call is
added (or a dangerous flag or option if those are deny-listed), and it
is often possible to alter the representation of a value without alter-
ing its meaning, leading to a deny-list bypass. See also Caveats be-
low.
The arch field is not unique for all calling conventions. The x86-64
ABI and the x32 ABI both use AUDIT_ARCH_X86_64 as arch, and they run on
the same processors. Instead, the mask __X32_SYSCALL_BIT is used on
the system call number to tell the two ABIs apart.
This means that a policy must either deny all syscalls with
__X32_SYSCALL_BIT or it must recognize syscalls with and without
__X32_SYSCALL_BIT set. A list of system calls to be denied based on nr
that does not also contain nr values with __X32_SYSCALL_BIT set can be
bypassed by a malicious program that sets __X32_SYSCALL_BIT.
Additionally, kernels prior to Linux 5.4 incorrectly permitted nr in
the ranges 512-547 as well as the corresponding non-x32 syscalls ORed
with __X32_SYSCALL_BIT. For example, nr == 521 and nr == (101 |
__X32_SYSCALL_BIT) would result in invocations of ptrace(2) with poten-
tially confused x32-vs-x86_64 semantics in the kernel. Policies in-
tended to work on kernels before Linux 5.4 must ensure that they deny
or otherwise correctly handle these system calls. On Linux 5.4 and
newer, such system calls will fail with the error ENOSYS, without doing
anything.
The instruction_pointer field provides the address of the machine-lan-
guage instruction that performed the system call. This might be useful
in conjunction with the use of /proc/pid/maps to perform checks based
on which region (mapping) of the program made the system call. (Proba-
bly, it is wise to lock down the mmap(2) and mprotect(2) system calls
to prevent the program from subverting such checks.)
When checking values from args, keep in mind that arguments are often
silently truncated before being processed, but after the seccomp check.
For example, this happens if the i386 ABI is used on an x86-64 kernel:
although the kernel will normally not look beyond the 32 lowest bits of
the arguments, the values of the full 64-bit registers will be present
in the seccomp data. A less surprising example is that if the x86-64
ABI is used to perform a system call that takes an argument of type
int, the more-significant half of the argument register is ignored by
the system call, but visible in the seccomp data.
A seccomp filter returns a 32-bit value consisting of two parts: the
most significant 16 bits (corresponding to the mask defined by the con-
stant SECCOMP_RET_ACTION_FULL) contain one of the "action" values
listed below; the least significant 16-bits (defined by the constant
SECCOMP_RET_DATA) are "data" to be associated with this return value.
If multiple filters exist, they are all executed, in reverse order of
their addition to the filter tree--that is, the most recently installed
filter is executed first. (Note that all filters will be called even
if one of the earlier filters returns SECCOMP_RET_KILL. This is done
to simplify the kernel code and to provide a tiny speed-up in the exe-
cution of sets of filters by avoiding a check for this uncommon case.)
The return value for the evaluation of a given system call is the
first-seen action value of highest precedence (along with its accompa-
nying data) returned by execution of all of the filters.
In decreasing order of precedence, the action values that may be re-
turned by a seccomp filter are:
SECCOMP_RET_KILL_PROCESS (since Linux 4.14)
This value results in immediate termination of the process, with
a core dump. The system call is not executed. By contrast with
SECCOMP_RET_KILL_THREAD below, all threads in the thread group
are terminated. (For a discussion of thread groups, see the de-
scription of the CLONE_THREAD flag in clone(2).)
The process terminates as though killed by a SIGSYS signal.
Even if a signal handler has been registered for SIGSYS, the
handler will be ignored in this case and the process always ter-
minates. To a parent process that is waiting on this process
(using waitpid(2) or similar), the returned wstatus will indi-
cate that its child was terminated as though by a SIGSYS signal.
SECCOMP_RET_KILL_THREAD (or SECCOMP_RET_KILL)
This value results in immediate termination of the thread that
made the system call. The system call is not executed. Other
threads in the same thread group will continue to execute.
The thread terminates as though killed by a SIGSYS signal. See
SECCOMP_RET_KILL_PROCESS above.
Before Linux 4.11, any process terminated in this way would not
trigger a coredump (even though SIGSYS is documented in sig-
nal(7) as having a default action of termination with a core
dump). Since Linux 4.11, a single-threaded process will dump
core if terminated in this way.
With the addition of SECCOMP_RET_KILL_PROCESS in Linux 4.14,
SECCOMP_RET_KILL_THREAD was added as a synonym for SEC-
COMP_RET_KILL, in order to more clearly distinguish the two ac-
tions.
Note: the use of SECCOMP_RET_KILL_THREAD to kill a single thread
in a multithreaded process is likely to leave the process in a
permanently inconsistent and possibly corrupt state.
SECCOMP_RET_TRAP
This value results in the kernel sending a thread-directed
SIGSYS signal to the triggering thread. (The system call is not
executed.) Various fields will be set in the siginfo_t struc-
ture (see sigaction(2)) associated with signal:
o si_signo will contain SIGSYS.
o si_call_addr will show the address of the system call in-
struction.
o si_syscall and si_arch will indicate which system call was
attempted.
o si_code will contain SYS_SECCOMP.
o si_errno will contain the SECCOMP_RET_DATA portion of the
filter return value.
The program counter will be as though the system call happened
(i.e., the program counter will not point to the system call in-
struction). The return value register will contain an architec-
ture-dependent value; if resuming execution, set it to something
appropriate for the system call. (The architecture dependency
is because replacing it with ENOSYS could overwrite some useful
information.)
SECCOMP_RET_ERRNO
This value results in the SECCOMP_RET_DATA portion of the fil-
ter's return value being passed to user space as the errno value
without executing the system call.
SECCOMP_RET_USER_NOTIF (since Linux 5.0)
Forward the system call to an attached user-space supervisor
process to allow that process to decide what to do with the sys-
tem call. If there is no attached supervisor (either because
the filter was not installed with the SECCOMP_FIL-
TER_FLAG_NEW_LISTENER flag or because the file descriptor was
closed), the filter returns ENOSYS (similar to what happens when
a filter returns SECCOMP_RET_TRACE and there is no tracer). See
seccomp_unotify(2) for further details.
Note that the supervisor process will not be notified if another
filter returns an action value with a precedence greater than
SECCOMP_RET_USER_NOTIF.
SECCOMP_RET_TRACE
When returned, this value will cause the kernel to attempt to
notify a ptrace(2)-based tracer prior to executing the system
call. If there is no tracer present, the system call is not ex-
ecuted and returns a failure status with errno set to ENOSYS.
A tracer will be notified if it requests PTRACE_O_TRACESECCOMP
using ptrace(PTRACE_SETOPTIONS). The tracer will be notified of
a PTRACE_EVENT_SECCOMP and the SECCOMP_RET_DATA portion of the
filter's return value will be available to the tracer via
PTRACE_GETEVENTMSG.
The tracer can skip the system call by changing the system call
number to -1. Alternatively, the tracer can change the system
call requested by changing the system call to a valid system
call number. If the tracer asks to skip the system call, then
the system call will appear to return the value that the tracer
puts in the return value register.
Before Linux 4.8, the seccomp check will not be run again after
the tracer is notified. (This means that, on older kernels,
seccomp-based sandboxes must not allow use of ptrace(2)--even of
other sandboxed processes--without extreme care; ptracers can
use this mechanism to escape from the seccomp sandbox.)
Note that a tracer process will not be notified if another fil-
ter returns an action value with a precedence greater than SEC-
COMP_RET_TRACE.
SECCOMP_RET_LOG (since Linux 4.14)
This value results in the system call being executed after the
filter return action is logged. An administrator may override
the logging of this action via the /proc/sys/kernel/seccomp/ac-
tions_logged file.
SECCOMP_RET_ALLOW
This value results in the system call being executed.
If an action value other than one of the above is specified, then the
filter action is treated as either SECCOMP_RET_KILL_PROCESS (since
Linux 4.14) or SECCOMP_RET_KILL_THREAD (in Linux 4.13 and earlier).
/proc interfaces
The files in the directory /proc/sys/kernel/seccomp provide additional
seccomp information and configuration:
actions_avail (since Linux 4.14)
A read-only ordered list of seccomp filter return actions in
string form. The ordering, from left-to-right, is in decreasing
order of precedence. The list represents the set of seccomp
filter return actions supported by the kernel.
actions_logged (since Linux 4.14)
A read-write ordered list of seccomp filter return actions that
are allowed to be logged. Writes to the file do not need to be
in ordered form but reads from the file will be ordered in the
same way as the actions_avail file.
It is important to note that the value of actions_logged does
not prevent certain filter return actions from being logged when
the audit subsystem is configured to audit a task. If the ac-
tion is not found in the actions_logged file, the final decision
on whether to audit the action for that task is ultimately left
up to the audit subsystem to decide for all filter return ac-
tions other than SECCOMP_RET_ALLOW.
The "allow" string is not accepted in the actions_logged file as
it is not possible to log SECCOMP_RET_ALLOW actions. Attempting
to write "allow" to the file will fail with the error EINVAL.
Audit logging of seccomp actions
Since Linux 4.14, the kernel provides the facility to log the actions
returned by seccomp filters in the audit log. The kernel makes the de-
cision to log an action based on the action type, whether or not the
action is present in the actions_logged file, and whether kernel audit-
ing is enabled (e.g., via the kernel boot option audit=1). The rules
are as follows:
o If the action is SECCOMP_RET_ALLOW, the action is not logged.
o Otherwise, if the action is either SECCOMP_RET_KILL_PROCESS or SEC-
COMP_RET_KILL_THREAD, and that action appears in the actions_logged
file, the action is logged.
o Otherwise, if the filter has requested logging (the SECCOMP_FIL-
TER_FLAG_LOG flag) and the action appears in the actions_logged
file, the action is logged.
o Otherwise, if kernel auditing is enabled and the process is being
audited (autrace(8)), the action is logged.
o Otherwise, the action is not logged.
RETURN VALUE
On success, seccomp() returns 0. On error, if SECCOMP_FIL-
TER_FLAG_TSYNC was used, the return value is the ID of the thread that
caused the synchronization failure. (This ID is a kernel thread ID of
the type returned by clone(2) and gettid(2).) On other errors, -1 is
returned, and errno is set to indicate the error.
ERRORS
seccomp() can fail for the following reasons:
EACCES The caller did not have the CAP_SYS_ADMIN capability in its user
namespace, or had not set no_new_privs before using SEC-
COMP_SET_MODE_FILTER.
EBUSY While installing a new filter, the SECCOMP_FILTER_FLAG_NEW_LIS-
TENER flag was specified, but a previous filter had already been
installed with that flag.
EFAULT args was not a valid address.
EINVAL operation is unknown or is not supported by this kernel version
or configuration.
EINVAL The specified flags are invalid for the given operation.
EINVAL operation included BPF_ABS, but the specified offset was not
aligned to a 32-bit boundary or exceeded sizeof(struct sec-
comp_data).
EINVAL A secure computing mode has already been set, and operation dif-
fers from the existing setting.
EINVAL operation specified SECCOMP_SET_MODE_FILTER, but the filter pro-
gram pointed to by args was not valid or the length of the fil-
ter program was zero or exceeded BPF_MAXINSNS (4096) instruc-
tions.
ENOMEM Out of memory.
ENOMEM The total length of all filter programs attached to the calling
thread would exceed MAX_INSNS_PER_PATH (32768) instructions.
Note that for the purposes of calculating this limit, each al-
ready existing filter program incurs an overhead penalty of 4
instructions.
EOPNOTSUPP
operation specified SECCOMP_GET_ACTION_AVAIL, but the kernel
does not support the filter return action specified by args.
ESRCH Another thread caused a failure during thread sync, but its ID
could not be determined.
STANDARDS
Linux.
HISTORY
Linux 3.17.
NOTES
Rather than hand-coding seccomp filters as shown in the example below,
you may prefer to employ the libseccomp library, which provides a
front-end for generating seccomp filters.
The Seccomp field of the /proc/pid/status file provides a method of
viewing the seccomp mode of a process; see proc(5).
seccomp() provides a superset of the functionality provided by the
prctl(2) PR_SET_SECCOMP operation (which does not support flags).
Since Linux 4.4, the ptrace(2) PTRACE_SECCOMP_GET_FILTER operation can
be used to dump a process's seccomp filters.
Architecture support for seccomp BPF
Architecture support for seccomp BPF filtering is available on the fol-
lowing architectures:
o x86-64, i386, x32 (since Linux 3.5)
o ARM (since Linux 3.8)
o s390 (since Linux 3.8)
o MIPS (since Linux 3.16)
o ARM-64 (since Linux 3.19)
o PowerPC (since Linux 4.3)
o Tile (since Linux 4.3)
o PA-RISC (since Linux 4.6)
Caveats
There are various subtleties to consider when applying seccomp filters
to a program, including the following:
o Some traditional system calls have user-space implementations in the
vdso(7) on many architectures. Notable examples include clock_get-
time(2), gettimeofday(2), and time(2). On such architectures, sec-
comp filtering for these system calls will have no effect. (How-
ever, there are cases where the vdso(7) implementations may fall
back to invoking the true system call, in which case seccomp filters
would see the system call.)
o Seccomp filtering is based on system call numbers. However, appli-
cations typically do not directly invoke system calls, but instead
call wrapper functions in the C library which in turn invoke the
system calls. Consequently, one must be aware of the following:
o The glibc wrappers for some traditional system calls may actually
employ system calls with different names in the kernel. For ex-
ample, the exit(2) wrapper function actually employs the
exit_group(2) system call, and the fork(2) wrapper function actu-
ally calls clone(2).
o The behavior of wrapper functions may vary across architectures,
according to the range of system calls provided on those archi-
tectures. In other words, the same wrapper function may invoke
different system calls on different architectures.
o Finally, the behavior of wrapper functions can change across
glibc versions. For example, in older versions, the glibc wrap-
per function for open(2) invoked the system call of the same
name, but starting in glibc 2.26, the implementation switched to
calling openat(2) on all architectures.
The consequence of the above points is that it may be necessary to fil-
ter for a system call other than might be expected. Various manual
pages in Section 2 provide helpful details about the differences be-
tween wrapper functions and the underlying system calls in subsections
entitled C library/kernel differences.
Furthermore, note that the application of seccomp filters even risks
causing bugs in an application, when the filters cause unexpected fail-
ures for legitimate operations that the application might need to per-
form. Such bugs may not easily be discovered when testing the seccomp
filters if the bugs occur in rarely used application code paths.
Seccomp-specific BPF details
Note the following BPF details specific to seccomp filters:
o The BPF_H and BPF_B size modifiers are not supported: all operations
must load and store (4-byte) words (BPF_W).
o To access the contents of the seccomp_data buffer, use the BPF_ABS
addressing mode modifier.
o The BPF_LEN addressing mode modifier yields an immediate mode oper-
and whose value is the size of the seccomp_data buffer.
EXAMPLES
The program below accepts four or more arguments. The first three ar-
guments are a system call number, a numeric architecture identifier,
and an error number. The program uses these values to construct a BPF
filter that is used at run time to perform the following checks:
o If the program is not running on the specified architecture, the BPF
filter causes system calls to fail with the error ENOSYS.
o If the program attempts to execute the system call with the speci-
fied number, the BPF filter causes the system call to fail, with er-
rno being set to the specified error number.
The remaining command-line arguments specify the pathname and addi-
tional arguments of a program that the example program should attempt
to execute using execv(3) (a library function that employs the ex-
ecve(2) system call). Some example runs of the program are shown be-
low.
First, we display the architecture that we are running on (x86-64) and
then construct a shell function that looks up system call numbers on
this architecture:
$ uname -m
x86_64
$ syscall_nr() {
cat /usr/src/linux/arch/x86/syscalls/syscall_64.tbl | \
awk '$2 != "x32" && $3 == "'$1'" { print $1 }'
}
When the BPF filter rejects a system call (case [2] above), it causes
the system call to fail with the error number specified on the command
line. In the experiments shown here, we'll use error number 99:
$ errno 99
EADDRNOTAVAIL 99 Cannot assign requested address
In the following example, we attempt to run the command whoami(1), but
the BPF filter rejects the execve(2) system call, so that the command
is not even executed:
$ syscall_nr execve
59
$ ./a.out
Usage: ./a.out <syscall_nr> <arch> <errno> <prog> [<args>]
Hint for <arch>: AUDIT_ARCH_I386: 0x40000003
AUDIT_ARCH_X86_64: 0xC000003E
$ ./a.out 59 0xC000003E 99 /bin/whoami
execv: Cannot assign requested address
In the next example, the BPF filter rejects the write(2) system call,
so that, although it is successfully started, the whoami(1) command is
not able to write output:
$ syscall_nr write
1
$ ./a.out 1 0xC000003E 99 /bin/whoami
In the final example, the BPF filter rejects a system call that is not
used by the whoami(1) command, so it is able to successfully execute
and produce output:
$ syscall_nr preadv
295
$ ./a.out 295 0xC000003E 99 /bin/whoami
cecilia
Program source
#include <linux/audit.h>
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/prctl.h>
#include <sys/syscall.h>
#include <unistd.h>
#define X32_SYSCALL_BIT 0x40000000
#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
static int
install_filter(int syscall_nr, unsigned int t_arch, int f_errno)
{
unsigned int upper_nr_limit = 0xffffffff;
/* Assume that AUDIT_ARCH_X86_64 means the normal x86-64 ABI
(in the x32 ABI, all system calls have bit 30 set in the
'nr' field, meaning the numbers are >= X32_SYSCALL_BIT). */
if (t_arch == AUDIT_ARCH_X86_64)
upper_nr_limit = X32_SYSCALL_BIT - 1;
struct sock_filter filter[] = {
/* [0] Load architecture from 'seccomp_data' buffer into
accumulator. */
BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
(offsetof(struct seccomp_data, arch))),
/* [1] Jump forward 5 instructions if architecture does not
match 't_arch'. */
BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, t_arch, 0, 5),
/* [2] Load system call number from 'seccomp_data' buffer into
accumulator. */
BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
(offsetof(struct seccomp_data, nr))),
/* [3] Check ABI - only needed for x86-64 in deny-list use
cases. Use BPF_JGT instead of checking against the bit
mask to avoid having to reload the syscall number. */
BPF_JUMP(BPF_JMP | BPF_JGT | BPF_K, upper_nr_limit, 3, 0),
/* [4] Jump forward 1 instruction if system call number
does not match 'syscall_nr'. */
BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, syscall_nr, 0, 1),
/* [5] Matching architecture and system call: don't execute
the system call, and return 'f_errno' in 'errno'. */
BPF_STMT(BPF_RET | BPF_K,
SECCOMP_RET_ERRNO | (f_errno & SECCOMP_RET_DATA)),
/* [6] Destination of system call number mismatch: allow other
system calls. */
BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),
/* [7] Destination of architecture mismatch: kill process. */
BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS),
};
struct sock_fprog prog = {
.len = ARRAY_SIZE(filter),
.filter = filter,
};
if (syscall(SYS_seccomp, SECCOMP_SET_MODE_FILTER, 0, &prog)) {
perror("seccomp");
return 1;
}
return 0;
}
int
main(int argc, char *argv[])
{
if (argc < 5) {
fprintf(stderr, "Usage: "
"%s <syscall_nr> <arch> <errno> <prog> [<args>]\n"
"Hint for <arch>: AUDIT_ARCH_I386: 0x%X\n"
" AUDIT_ARCH_X86_64: 0x%X\n"
"\n", argv[0], AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
exit(EXIT_FAILURE);
}
if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
perror("prctl");
exit(EXIT_FAILURE);
}
if (install_filter(strtol(argv[1], NULL, 0),
strtoul(argv[2], NULL, 0),
strtol(argv[3], NULL, 0)))
exit(EXIT_FAILURE);
execv(argv[4], &argv[4]);
perror("execv");
exit(EXIT_FAILURE);
}
SEE ALSO
bpfc(1), strace(1), bpf(2), prctl(2), ptrace(2), seccomp_unotify(2),
sigaction(2), proc(5), signal(7), socket(7)
Various pages from the libseccomp library, including: scmp_sys_re-
solver(1), seccomp_export_bpf(3), seccomp_init(3), seccomp_load(3), and
seccomp_rule_add(3).
The kernel source files Documentation/networking/filter.txt and Docu-
mentation/userspace-api/seccomp_filter.rst (or Documentation/prctl/sec-
comp_filter.txt before Linux 4.13).
McCanne, S. and Jacobson, V. (1992) The BSD Packet Filter: A New Archi-
tecture for User-level Packet Capture, Proceedings of the USENIX Winter
1993 Conference <http://www.tcpdump.org/papers/bpf-usenix93.pdf>
Linux man-pages 6.04 2023-03-30 seccomp(2)
Want to link to this manual page? Use this URL:
<http://star2.abcm.com/cgi-bin/bsdi-man?query=seccomp&sektion=2&manpath=>