работать с состоянием безопасных вычислений процесса (operate on Secure Computing state of the process)
Имя (Name)
seccomp - operate on Secure Computing state of the process
Синопсис (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).
Описание (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
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 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
returns 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 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 because it did not actually drop
privileges.)
If prctl(2) or seccomp() 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_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_FILTER_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
operation is helpful to confirm that the kernel knows of a
more recently 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 command 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
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 (and the
convention being used may vary over the life of a process that
uses execve(2) to execute binaries 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
possible 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 altering its meaning, leading
to a deny-list bypass. See also Caveats below.
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 potentially confused x32-vs-x86_64 semantics in
the kernel. Policies intended 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-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, 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_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 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 action
value of highest precedence (along with its accompanying data)
returned by execution of all of the filters.
In decreasing order of precedence, the action values that may be
returned 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 description 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 terminates. To a parent process that is
waiting on this process (using waitpid(2) or similar), the
returned wstatus will indicate 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 signal(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
SECCOMP_RET_KILL, in order to more clearly distinguish the
two actions.
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 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., the program counter 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_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 system call. If there is no attached
supervisor (either because the filter was not installed
with the SECCOMP_FILTER_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 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.
Before kernel 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
filter returns an action value with a precedence greater
than SECCOMP_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/actions_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 action 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 actions 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 decision to log an action based on the action type,
whether or not the action is present in the actions_logged file,
and whether kernel auditing is enabled (e.g., via the kernel boot
option audit=1). The rules are as follows:
* If the action is SECCOMP_RET_ALLOW, the action is not logged.
* Otherwise, if the action is either SECCOMP_RET_KILL_PROCESS or
SECCOMP_RET_KILL_THREAD, and that action appears in the
actions_logged file, the action is logged.
* Otherwise, if the filter has requested logging (the
SECCOMP_FILTER_FLAG_LOG flag) and the action appears in the
actions_logged file, the action is logged.
* Otherwise, if kernel auditing is enabled and the process is
being audited (autrace(8)), the action is logged.
* Otherwise, the action is not logged.
