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NAME
seccomp - operate on Secure Computing state of the process
SYNOPSIS
#include <linux/seccomp.h> #include <linux/filter.h> #include <linux/audit.h> #include <linux/signal.h> #include <sys/ptrace.h> int seccomp(unsigned int operation, unsigned int flags, void *args);
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 delivery of a SIGKILL
signal. 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 sigprocmask(2), it can use sigreturn(2)
to block all signals apart from SIGKILL and SIGSTOP. This means that alarm(2) (for example) 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 setrlimit(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 invalid, seccomp() fails, returning
EINVAL in errno.
If fork(2) or clone(2) is allowed by the filter, any child processes 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 caller must have the
CAP_SYS_ADMIN capability, 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 will fail and return EACCES in errno. This
requirement ensures that an unprivileged 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 non-zero 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 because it did not actually drop
privileges.)
If prctl(2) or seccomp(2) is allowed by the attached filter, further 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_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.
Filters
When adding filters via SECCOMP_SET_MODE_FILTER, args points to a filter 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, it is usually necessary
to verify the value of the arch field.
It is strongly recommended to use a whitelisting approach whenever possible because such an approach is
more robust and simple. A blacklist will have to be updated whenever a potentially dangerous system call
is added (or a dangerous flag or option if those are blacklisted), and it is often possible to alter the
representation of a value without altering its meaning, leading to a blacklist bypass.
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 in order to create a seccomp-based blacklist for system calls performed through the
x86-64 ABI, it is necessary to not only check that arch equals AUDIT_ARCH_X86_64, but also to explicitly
reject all system calls that contain __X32_SYSCALL_BIT in nr.
The instruction_pointer field provides the address of the machine-language 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. (Probably, 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 against a blacklist, 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 constant SECCOMP_RET_ACTION) 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 execution 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 SECCOMP_RET_ACTION
value of highest precedence (along with its accompanying data) returned by execution of all of the
filters.
In decreasing order of precedence, the values that may be returned by a seccomp filter are:
SECCOMP_RET_KILL
This value results in the process exiting immediately without executing the system call. The
process terminates as though killed by a SIGSYS signal (not SIGKILL).
SECCOMP_RET_TRAP
This value results in the kernel sending a SIGSYS signal to the triggering process without
executing the system call. Various fields will be set in the siginfo_t structure (see
sigaction(2)) associated with signal:
* si_signo will contain SIGSYS.
* si_call_addr will show the address of the system call instruction.
* si_syscall and si_arch will indicate which system call was attempted.
* si_code will contain SYS_SECCOMP.
* 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., it will not point to the
system call instruction). The return value register will contain an architecture-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 filter's return value being passed to
user space as the errno value without executing the system call.
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
executed 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.
The seccomp check will not be run again after the tracer is notified. (This means that 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.)
SECCOMP_RET_ALLOW
This value results in the system call being executed.
RETURN VALUE
On success, seccomp() returns 0. On error, if SECCOMP_FILTER_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
cause of the error.
ERRORS
seccomp() can fail for the following reasons:
EACCESS
The caller did not have the CAP_SYS_ADMIN capability, or had not set no_new_privs before using
SECCOMP_SET_MODE_FILTER.
EFAULT args was not a valid address.
EINVAL operation is unknown; or 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 seccomp_data).
EINVAL A secure computing mode has already been set, and operation differs from the existing setting.
EINVAL operation specified SECCOMP_SET_MODE_FILTER, but the kernel was not built with
CONFIG_SECCOMP_FILTER enabled.
EINVAL operation specified SECCOMP_SET_MODE_FILTER, but the filter program pointed to by args was not
valid or the length of the filter program was zero or exceeded BPF_MAXINSNS (4096) instructions.
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 already existing filter program incurs an overhead penalty of 4 instructions.
ESRCH Another thread caused a failure during thread sync, but its ID could not be determined.
VERSIONS
The seccomp() system call first appeared in Linux 3.17.
CONFORMING TO
The seccomp() system call is a nonstandard Linux extension.
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).
Seccomp-specific BPF details
Note the following BPF details specific to seccomp filters:
* The BPF_H and BPF_B size modifiers are not supported: all operations must load and store (4-byte)
words (BPF_W).
* To access the contents of the seccomp_data buffer, use the BPF_ABS addressing mode modifier.
* The BPF_LEN addressing mode modifier yields an immediate mode operand whose value is the size of the
seccomp_data buffer.
EXAMPLE
The program below accepts four or more arguments. The first three arguments 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:
[1] If the program is not running on the specified architecture, the BPF filter causes system calls to
fail with the error ENOSYS.
[2] If the program attempts to execute the system call with the specified number, the BPF filter causes
the system call to fail, with errno being set to the specified error number.
The remaining command-line arguments specify the pathname and additional arguments of a program that the
example program should attempt to execute using execv(3) (a library function that employs the execve(2)
system call). Some example runs of the program are shown below.
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 <errno.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <linux/audit.h>
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <sys/prctl.h>
#define X32_SYSCALL_BIT 0x40000000
static int
install_filter(int syscall_nr, int t_arch, int f_errno)
{
unsigned int upper_nr_limit = 0xffffffff;
/* Assume that AUDIT_ARCH_X86_64 means the normal x86-64 ABI */
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 blacklist use
cases. Use 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),
};
struct sock_fprog prog = {
.len = (unsigned short) (sizeof(filter) / sizeof(filter[0])),
.filter = filter,
};
if (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),
strtol(argv[2], NULL, 0),
strtol(argv[3], NULL, 0)))
exit(EXIT_FAILURE);
execv(argv[4], &argv[4]);
perror("execv");
exit(EXIT_FAILURE);
}
SEE ALSO
bpf(2), prctl(2), ptrace(2), sigaction(2), proc(5), signal(7), socket(7) Various pages from the libseccomp library, including: scmp_sys_resolver(1), seccomp_init(3), seccomp_load(3), seccomp_rule_add(3), and seccomp_export_bpf(3). The kernel source files Documentation/networking/filter.txt and Documentation/prctl/seccomp_filter.txt. McCanne, S. and Jacobson, V. (1992) The BSD Packet Filter: A New Architecture for User-level Packet Capture, Proceedings of the USENIX Winter 1993 Conference ⟨http://www.tcpdump.org/papers/bpf- usenix93.pdf⟩
COLOPHON
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