proc.5: Note kernel version for /proc/PID/smaps VmFlags "wf" flag

[thirdparty/man-pages.git] / man2 / seccomp.2

.\" Copyright (C) 2014 Kees Cook <keescook@chromium.org>
.\" and Copyright (C) 2012 Will Drewry <wad@chromium.org>
.\" and Copyright (C) 2008, 2014,2017 Michael Kerrisk <mtk.manpages@gmail.com>
.\" and Copyright (C) 2017 Tyler Hicks <tyhicks@canonical.com>
.\"
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.\"
.TH SECCOMP 2 2019-11-19 "Linux" "Linux Programmer's Manual"
.SH NAME
seccomp \- operate on Secure Computing state of the process
.SH SYNOPSIS
.nf
.B #include <linux/seccomp.h>
.B #include <linux/filter.h>
.B #include <linux/audit.h>
.B #include <linux/signal.h>
.B #include <sys/ptrace.h>
.\" Kees Cook noted: Anything that uses SECCOMP_RET_TRACE returns will
.\"                  need <sys/ptrace.h>
.PP
.BI "int seccomp(unsigned int " operation ", unsigned int " flags \
", void *" args );
.fi
.SH DESCRIPTION
The
.BR seccomp ()
system call operates on the Secure Computing (seccomp) state of the
calling process.
.PP
Currently, Linux supports the following
.IR operation
values:
.TP
.BR SECCOMP_SET_MODE_STRICT
The only system calls that the calling thread is permitted to make are
.BR read (2),
.BR write (2),
.BR _exit (2)
(but not
.BR exit_group (2)),
and
.BR sigreturn (2).
Other system calls result in the delivery of a
.BR 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.
.IP
Note that although the calling thread can no longer call
.BR sigprocmask (2),
it can use
.BR sigreturn (2)
to block all signals apart from
.BR SIGKILL
and
.BR SIGSTOP .
This means that
.BR alarm (2)
(for example) is not sufficient for restricting the process's execution time.
Instead, to reliably terminate the process,
.BR SIGKILL
must be used.
This can be done by using
.BR timer_create (2)
with
.BR SIGEV_SIGNAL
and
.IR sigev_signo
set to
.BR SIGKILL ,
or by using
.BR setrlimit (2)
to set the hard limit for
.BR RLIMIT_CPU .
.IP
This operation is available only if the kernel is configured with
.BR CONFIG_SECCOMP
enabled.
.IP
The value of
.IR flags
must be 0, and
.IR args
must be NULL.
.IP
This operation is functionally identical to the call:
.IP
.in +4n
.EX
prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);
.EE
.in
.TP
.BR SECCOMP_SET_MODE_FILTER
The system calls allowed are defined by a pointer to a Berkeley Packet
Filter (BPF) passed via
.IR args .
This argument is a pointer to a
.IR "struct\ sock_fprog" ;
it can be designed to filter arbitrary system calls and system call
arguments.
If the filter is invalid,
.BR seccomp ()
fails, returning
.BR EINVAL
in
.IR errno .
.IP
If
.BR fork (2)
or
.BR clone (2)
is allowed by the filter, any child processes will be constrained to
the same system call filters as the parent.
If
.BR execve (2)
is allowed,
the existing filters will be preserved across a call to
.BR execve (2).
.IP
In order to use the
.BR SECCOMP_SET_MODE_FILTER
operation, either the calling thread must have the
.BR CAP_SYS_ADMIN
capability in its user namespace, or the thread must already have the
.I 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:
.IP
.in +4n
.EX
prctl(PR_SET_NO_NEW_PRIVS, 1);
.EE
.in
.IP
Otherwise, the
.BR SECCOMP_SET_MODE_FILTER
operation fails and returns
.BR EACCES
in
.IR 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
.BR execve (2),
thus potentially compromising that program.
(Such a malicious filter might, for example, cause an attempt to use
.BR 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.)
.IP
If
.BR prctl (2)
or
.BR 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.
.IP
The
.BR SECCOMP_SET_MODE_FILTER
operation is available only if the kernel is configured with
.BR CONFIG_SECCOMP_FILTER
enabled.
.IP
When
.IR flags
is 0, this operation is functionally identical to the call:
.IP
.in +4n
.EX
prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, args);
.EE
.in
.IP
The recognized
.IR flags
are:
.RS
.TP
.BR 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
.BR seccomp ()
calls results in different filters from this perspective.)
.IP
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
.BR SECCOMP_MODE_STRICT
or if it has attached new seccomp filters to itself,
diverging from the calling thread's filter tree.
.TP
.BR SECCOMP_FILTER_FLAG_LOG " (since Linux 4.14)"
.\" commit e66a39977985b1e69e17c4042cb290768eca9b02
All filter return actions except
.BR SECCOMP_RET_ALLOW
should be logged.
An administrator may override this filter flag by preventing specific
actions from being logged via the
.IR /proc/sys/kernel/seccomp/actions_logged
file.
.TP
.BR SECCOMP_FILTER_FLAG_SPEC_ALLOW " (since Linux 4.17)"
.\" commit 00a02d0c502a06d15e07b857f8ff921e3e402675
Disable Speculative Store Bypass mitigation.
.RE
.TP
.BR SECCOMP_GET_ACTION_AVAIL " (since Linux 4.14)"
.\" commit d612b1fd8010d0d67b5287fe146b8b55bcbb8655
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
.BR SECCOMP_RET_KILL_PROCESS .
.IP
The value of
.IR flags
must be 0, and
.IR args
must be a pointer to an unsigned 32-bit filter return action.
.SS Filters
When adding filters via
.BR SECCOMP_SET_MODE_FILTER ,
.IR args
points to a filter program:
.PP
.in +4n
.EX
struct sock_fprog {
    unsigned short      len;    /* Number of BPF instructions */
    struct sock_filter *filter; /* Pointer to array of
                                   BPF instructions */
};
.EE
.in
.PP
Each program must contain one or more BPF instructions:
.PP
.in +4n
.EX
struct sock_filter {            /* Filter block */
    __u16 code;                 /* Actual filter code */
    __u8  jt;                   /* Jump true */
    __u8  jf;                   /* Jump false */
    __u32 k;                    /* Generic multiuse field */
};
.EE
.in
.PP
When executing the instructions, the BPF program operates on the
system call information made available (i.e., use the
.BR BPF_ABS
addressing mode) as a (read-only)
.\" Quoting Kees Cook:
.\"     If BPF even allows changing the data, it's not copied back to
.\"     the syscall when it runs. Anything wanting to do things like
.\"     that would need to use ptrace to catch the call and directly
.\"     modify the registers before continuing with the call.
buffer of the following form:
.PP
.in +4n
.EX
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 */
};
.EE
.in
.PP
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
.BR execve (2)
to execute binaries that employ the different conventions),
it is usually necessary to verify the value of the
.IR arch
field.
.PP
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
.IR Caveats
below.
.PP
The
.IR arch
field is not unique for all calling conventions.
The x86-64 ABI and the x32 ABI both use
.BR AUDIT_ARCH_X86_64
as
.IR arch ,
and they run on the same processors.
Instead, the mask
.BR __X32_SYSCALL_BIT
is used on the system call number to tell the two ABIs apart.
.\" As noted by Dave Drysdale in a note at the end of
.\" https://lwn.net/Articles/604515/
.\"     One additional detail to point out for the x32 ABI case:
.\"     the syscall number gets a high bit set (__X32_SYSCALL_BIT),
.\"     to mark it as an x32 call.
.\"
.\"     If x32 support is included in the kernel, then __SYSCALL_MASK
.\"     will have a value that is not all-ones, and this will trigger
.\"     an extra instruction in system_call to mask off the extra bit,
.\"     so that the syscall table indexing still works.
.PP
This means that in order to create a seccomp-based
deny-list for system calls performed through the x86-64 ABI,
it is necessary to not only check that
.IR arch
equals
.BR AUDIT_ARCH_X86_64 ,
but also to explicitly reject all system calls that contain
.BR __X32_SYSCALL_BIT
in
.IR nr .
.PP
The
.I 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
.I /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
.BR mmap (2)
and
.BR mprotect (2)
system calls to prevent the program from subverting such checks.)
.PP
When checking values from
.IR args
against a deny-list, 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
.IR int ,
the more-significant half of the argument register is ignored by
the system call, but visible in the seccomp data.
.PP
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
.BR SECCOMP_RET_ACTION_FULL )
contain one of the "action" values listed below;
the least significant 16-bits (defined by the constant
.BR SECCOMP_RET_DATA )
are "data" to be associated with this return value.
.PP
If multiple filters exist, they are \fIall\fP executed,
in reverse order of their addition to the filter tree\(emthat is,
the most recently installed filter is executed first.
(Note that all filters will be called
even if one of the earlier filters returns
.BR 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.)
.\" From an Aug 2015 conversation with Kees Cook where I asked why *all*
.\" filters are applied even if one of the early filters returns
.\" SECCOMP_RET_KILL:
.\"
.\"     It's just because it would be an optimization that would only speed up
.\"     the RET_KILL case, but it's the uncommon one and the one that doesn't
.\"     benefit meaningfully from such a change (you need to kill the process
.\"     really quickly?). We would speed up killing a program at the (albeit
.\"     tiny) expense to all other filtered programs. Best to keep the filter
.\"     execution logic clear, simple, and as fast as possible for all
.\"     filters.
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.
.PP
In decreasing order of precedence,
the action values that may be returned by a seccomp filter are:
.TP
.BR SECCOMP_RET_KILL_PROCESS " (since Linux 4.14)"
.\" commit 4d3b0b05aae9ee9ce0970dc4cc0fb3fad5e85945
.\" commit 0466bdb99e8744bc9befa8d62a317f0fd7fd7421
This value results in immediate termination of the process,
with a core dump.
The system call is not executed.
By contrast with
.BR SECCOMP_RET_KILL_THREAD
below, all threads in the thread group are terminated.
(For a discussion of thread groups, see the description of the
.BR CLONE_THREAD
flag in
.BR clone (2).)
.IP
The process terminates
.I "as though"
killed by a
.B SIGSYS
signal.
Even if a signal handler has been registered for
.BR 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
.BR waitpid (2)
or similar), the returned
.I wstatus
will indicate that its child was terminated as though by a
.BR SIGSYS
signal.
.TP
.BR 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.
.IP
The thread terminates
.I "as though"
killed by a
.B SIGSYS
signal.
See
.BR SECCOMP_RET_KILL_PROCESS
above.
.IP
.\" See these commits:
.\" seccomp: dump core when using SECCOMP_RET_KILL
.\"    (b25e67161c295c98acda92123b2dd1e7d8642901)
.\" seccomp: Only dump core when single-threaded
.\"    (d7276e321ff8a53106a59c85ca46d03e34288893)
Before Linux 4.11,
any process terminated in this way would not trigger a coredump
(even though
.B SIGSYS
is documented in
.BR 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.
.IP
With the addition of
.BR SECCOMP_RET_KILL_PROCESS
in Linux 4.14,
.BR SECCOMP_RET_KILL_THREAD
was added as a synonym for
.BR SECCOMP_RET_KILL ,
in order to more clearly distinguish the two actions.
.TP
.BR SECCOMP_RET_TRAP
This value results in the kernel sending a thread-directed
.BR SIGSYS
signal to the triggering thread.
(The system call is not executed.)
Various fields will be set in the
.I siginfo_t
structure (see
.BR sigaction (2))
associated with signal:
.RS
.IP * 3
.I si_signo
will contain
.BR SIGSYS .
.IP *
.IR si_call_addr
will show the address of the system call instruction.
.IP *
.IR si_syscall
and
.IR si_arch
will indicate which system call was attempted.
.IP *
.I si_code
will contain
.BR SYS_SECCOMP .
.IP *
.I si_errno
will contain the
.BR SECCOMP_RET_DATA
portion of the filter return value.
.RE
.IP
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
.BR ENOSYS
could overwrite some useful information.)
.TP
.BR SECCOMP_RET_ERRNO
This value results in the
.B SECCOMP_RET_DATA
portion of the filter's return value being passed to user space as the
.IR errno
value without executing the system call.
.TP
.BR SECCOMP_RET_TRACE
When returned, this value will cause the kernel to attempt to notify a
.BR 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
.I errno
set to
.BR ENOSYS .
.IP
A tracer will be notified if it requests
.BR PTRACE_O_TRACESECCOMP
using
.IR ptrace(PTRACE_SETOPTIONS) .
The tracer will be notified of a
.BR PTRACE_EVENT_SECCOMP
and the
.BR SECCOMP_RET_DATA
portion of the filter's return value will be available to the tracer via
.BR PTRACE_GETEVENTMSG .
.IP
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.
.IP
.\" This was changed in ce6526e8afa4.
.\" A related hole, using PTRACE_SYSCALL instead of SECCOMP_RET_TRACE, was
.\" changed in arch-specific commits, e.g. 93e35efb8de4 for X86 and
.\" 0f3912fd934c for ARM.
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
.B "must not"
allow use of
.BR ptrace (2)\(emeven
of other
sandboxed processes\(emwithout extreme care;
ptracers can use this mechanism to escape from the seccomp sandbox.)
.TP
.BR SECCOMP_RET_LOG " (since Linux 4.14)"
.\" commit 59f5cf44a38284eb9e76270c786fb6cc62ef8ac4
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
.IR /proc/sys/kernel/seccomp/actions_logged
file.
.TP
.BR SECCOMP_RET_ALLOW
This value results in the system call being executed.
.PP
If an action value other than one of the above is specified,
then the filter action is treated as either
.BR SECCOMP_RET_KILL_PROCESS
(since Linux 4.14)
.\" commit 4d3b0b05aae9ee9ce0970dc4cc0fb3fad5e85945
or
.BR SECCOMP_RET_KILL_THREAD
(in Linux 4.13 and earlier).
.\"
.SS /proc interfaces
The files in the directory
.IR /proc/sys/kernel/seccomp
provide additional seccomp information and configuration:
.TP
.IR actions_avail " (since Linux 4.14)"
.\" commit 8e5f1ad116df6b0de65eac458d5e7c318d1c05af
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.
.TP
.IR actions_logged " (since Linux 4.14)"
.\" commit 0ddec0fc8900201c0897b87b762b7c420436662f
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
.IR actions_avail
file.
.IP
It is important to note that the value of
.IR 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
.IR 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
.BR SECCOMP_RET_ALLOW .
.IP
The "allow" string is not accepted in the
.IR actions_logged
file as it is not possible to log
.BR SECCOMP_RET_ALLOW
actions.
Attempting to write "allow" to the file will fail with the error
.BR EINVAL .
.\"
.SS Audit logging of seccomp actions
.\" commit 59f5cf44a38284eb9e76270c786fb6cc62ef8ac4
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
.I actions_logged
file, and whether kernel auditing is enabled
(e.g., via the kernel boot option
.IR audit=1 ).
.\" or auditing could be enabled via the netlink API (AUDIT_SET)
The rules are as follows:
.IP * 3
If the action is
.BR SECCOMP_RET_ALLOW ,
the action is not logged.
.IP *
Otherwise, if the action is either
.BR SECCOMP_RET_KILL_PROCESS
or
.BR SECCOMP_RET_KILL_THREAD ,
and that action appears in the
.IR actions_logged
file, the action is logged.
.IP *
Otherwise, if the filter has requested logging (the
.BR SECCOMP_FILTER_FLAG_LOG
flag)
and the action appears in the
.IR actions_logged
file, the action is logged.
.IP *
Otherwise, if kernel auditing is enabled and the process is being audited
.RB ( autrace (8)),
the action is logged.
.IP *
Otherwise, the action is not logged.
.SH RETURN VALUE
On success,
.BR seccomp ()
returns 0.
On error, if
.BR 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
.BR clone (2)
and
.BR gettid (2).)
On other errors, \-1 is returned, and
.IR errno
is set to indicate the cause of the error.
.SH ERRORS
.BR seccomp ()
can fail for the following reasons:
.TP
.BR EACCES
The caller did not have the
.BR CAP_SYS_ADMIN
capability in its user namespace, or had not set
.IR no_new_privs
before using
.BR SECCOMP_SET_MODE_FILTER .
.TP
.BR EFAULT
.IR args
was not a valid address.
.TP
.BR EINVAL
.IR operation
is unknown or is not supported by this kernel version or configuration.
.TP
.B EINVAL
The specified
.IR flags
are invalid for the given
.IR operation .
.TP
.BR EINVAL
.I operation
included
.BR BPF_ABS ,
but the specified offset was not aligned to a 32-bit boundary or exceeded
.IR "sizeof(struct\ seccomp_data)" .
.TP
.BR EINVAL
.\" See kernel/seccomp.c::seccomp_may_assign_mode() in 3.18 sources
A secure computing mode has already been set, and
.I operation
differs from the existing setting.
.TP
.BR EINVAL
.I operation
specified
.BR SECCOMP_SET_MODE_FILTER ,
but the filter program pointed to by
.I args
was not valid or the length of the filter program was zero or exceeded
.B BPF_MAXINSNS
(4096) instructions.
.TP
.BR ENOMEM
Out of memory.
.TP
.BR ENOMEM
.\" ENOMEM in kernel/seccomp.c::seccomp_attach_filter() in 3.18 sources
The total length of all filter programs attached
to the calling thread would exceed
.B 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.
.TP
.BR EOPNOTSUPP
.I operation
specified
.BR SECCOMP_GET_ACTION_AVAIL ,
but the kernel does not support the filter return action specified by
.IR args .
.TP
.BR ESRCH
Another thread caused a failure during thread sync, but its ID could not
be determined.
.SH VERSIONS
The
.BR seccomp ()
system call first appeared in Linux 3.17.
.\" FIXME . Add glibc version
.SH CONFORMING TO
The
.BR seccomp ()
system call is a nonstandard Linux extension.
.SH NOTES
Rather than hand-coding seccomp filters as shown in the example below,
you may prefer to employ the
.I libseccomp
library, which provides a front-end for generating seccomp filters.
.PP
The
.IR Seccomp
field of the
.IR /proc/[pid]/status
file provides a method of viewing the seccomp mode of a process; see
.BR proc (5).
.PP
.BR seccomp ()
provides a superset of the functionality provided by the
.BR prctl (2)
.BR PR_SET_SECCOMP
operation (which does not support
.IR flags ).
.PP
Since Linux 4.4, the
.BR ptrace (2)
.B PTRACE_SECCOMP_GET_FILTER
operation can be used to dump a process's seccomp filters.
.\"
.SS Architecture support for seccomp BPF
Architecture support for seccomp BPF filtering
.\" Check by grepping for HAVE_ARCH_SECCOMP_FILTER in Kconfig files in
.\" kernel source. Last checked in Linux 4.16-rc source.
is available on the following architectures:
.IP * 3
x86-64, i386, x32 (since Linux 3.5)
.PD 0
.IP *
ARM (since Linux 3.8)
.IP *
s390 (since Linux 3.8)
.IP *
MIPS (since Linux 3.16)
.IP *
ARM-64 (since Linux 3.19)
.IP *
PowerPC (since Linux 4.3)
.IP *
Tile (since Linux 4.3)
.IP *
PA-RISC (since Linux 4.6)
.\" User mode Linux since Linux 4.6
.PD
.\"
.SS Caveats
There are various subtleties to consider when applying seccomp filters
to a program, including the following:
.IP * 3
Some traditional system calls have user-space implementations in the
.BR vdso (7)
on many architectures.
Notable examples include
.BR clock_gettime (2),
.BR gettimeofday (2),
and
.BR time (2).
On such architectures,
seccomp filtering for these system calls will have no effect.
(However, there are cases where the
.BR vdso (7)
implementations may fall back to invoking the true system call,
in which case seccomp filters would see the system call.)
.IP *
Seccomp filtering is based on system call numbers.
However, applications 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:
.RS
.IP \(bu 3
The glibc wrappers for some traditional system calls may actually
employ system calls with different names in the kernel.
For example, the
.BR exit (2)
wrapper function actually employs the
.BR exit_group (2)
system call, and the
.BR fork (2)
wrapper function actually calls
.BR clone (2).
.IP \(bu
The behavior of wrapper functions may vary across architectures,
according to the range of system calls provided on those architectures.
In other words, the same wrapper function may invoke
different system calls on different architectures.
.IP \(bu
Finally, the behavior of wrapper functions can change across glibc versions.
For example, in older versions, the glibc wrapper function for
.BR open (2)
invoked the system call of the same name,
but starting in glibc 2.26, the implementation switched to calling
.BR openat (2)
on all architectures.
.RE
.PP
The consequence of the above points is that it may be necessary
to filter for a system call other than might be expected.
Various manual pages in Section 2 provide helpful details
about the differences between wrapper functions and
the underlying system calls in subsections entitled
.IR "C library/kernel differences" .
.PP
Furthermore, note that the application of seccomp filters
even risks causing bugs in an application,
when the filters cause unexpected failures for legitimate operations
that the application might need to perform.
Such bugs may not easily be discovered when testing the seccomp
filters if the bugs occur in rarely used application code paths.
.\"
.SS Seccomp-specific BPF details
Note the following BPF details specific to seccomp filters:
.IP * 3
The
.B BPF_H
and
.B BPF_B
size modifiers are not supported: all operations must load and store
(4-byte) words
.RB ( BPF_W ).
.IP *
To access the contents of the
.I seccomp_data
buffer, use the
.B BPF_ABS
addressing mode modifier.
.IP *
The
.B BPF_LEN
addressing mode modifier yields an immediate mode operand
whose value is the size of the
.IR seccomp_data
buffer.
.SH 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:
.IP [1] 4
If the program is not running on the specified architecture,
the BPF filter causes system calls to fail with the error
.BR ENOSYS .
.IP [2]
If the program attempts to execute the system call with the specified number,
the BPF filter causes the system call to fail, with
.I errno
being set to the specified error number.
.PP
The remaining command-line arguments specify
the pathname and additional arguments of a program
that the example program should attempt to execute using
.BR execv (3)
(a library function that employs the
.BR execve (2)
system call).
Some example runs of the program are shown below.
.PP
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:
.PP
.in +4n
.EX
$ \fBuname -m\fP
x86_64
$ \fBsyscall_nr() {
    cat /usr/src/linux/arch/x86/syscalls/syscall_64.tbl | \e
    awk '$2 != "x32" && $3 == "'$1'" { print $1 }'
}\fP
.EE
.in
.PP
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:
.PP
.in +4n
.EX
$ \fBerrno 99\fP
EADDRNOTAVAIL 99 Cannot assign requested address
.EE
.in
.PP
In the following example, we attempt to run the command
.BR whoami (1),
but the BPF filter rejects the
.BR execve (2)
system call, so that the command is not even executed:
.PP
.in +4n
.EX
$ \fBsyscall_nr execve\fP
59
$ \fB./a.out\fP
Usage: ./a.out <syscall_nr> <arch> <errno> <prog> [<args>]
Hint for <arch>: AUDIT_ARCH_I386: 0x40000003
                 AUDIT_ARCH_X86_64: 0xC000003E
$ \fB./a.out 59 0xC000003E 99 /bin/whoami\fP
execv: Cannot assign requested address
.EE
.in
.PP
In the next example, the BPF filter rejects the
.BR write (2)
system call, so that, although it is successfully started, the
.BR whoami (1)
command is not able to write output:
.PP
.in +4n
.EX
$ \fBsyscall_nr write\fP
1
$ \fB./a.out 1 0xC000003E 99 /bin/whoami\fP
.EE
.in
.PP
In the final example,
the BPF filter rejects a system call that is not used by the
.BR whoami (1)
command, so it is able to successfully execute and produce output:
.PP
.in +4n
.EX
$ \fBsyscall_nr preadv\fP
295
$ \fB./a.out 295 0xC000003E 99 /bin/whoami\fP
cecilia
.EE
.in
.SS Program source
.EX
#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
       (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 task */
        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>]\en"
                "Hint for <arch>: AUDIT_ARCH_I386: 0x%X\en"
                "                 AUDIT_ARCH_X86_64: 0x%X\en"
                "\en", 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);
}
.EE
.SH SEE ALSO
.BR bpfc (1),
.BR strace (1),
.BR bpf (2),
.BR prctl (2),
.BR ptrace (2),
.BR sigaction (2),
.BR proc (5),
.BR signal (7),
.BR socket (7)
.PP
Various pages from the
.I libseccomp
library, including:
.BR scmp_sys_resolver (1),
.BR seccomp_export_bpf (3),
.BR seccomp_init (3),
.BR seccomp_load (3),
and
.BR seccomp_rule_add (3).
.PP
The kernel source files
.IR Documentation/networking/filter.txt
and
.IR Documentation/userspace\-api/seccomp_filter.rst
.\" commit c061f33f35be0ccc80f4b8e0aea5dfd2ed7e01a3
(or
.IR Documentation/prctl/seccomp_filter.txt
before Linux 4.13).
.PP
McCanne, S.\& and Jacobson, V.\& (1992)
.IR "The BSD Packet Filter: A New Architecture for User-level Packet Capture" ,
Proceedings of the USENIX Winter 1993 Conference
.UR http://www.tcpdump.org/papers/bpf\-usenix93.pdf
.UE