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SubjectEdited seccomp.2 man page for review [v2]
Hi Kees, (and all),

Thanks for your comments on the previous draft of the seccomp(2)
man page and (once again) my apologies for the slow follow-up.

I have done some further editing of the page. Could you check
the revised version below. I have added a number of FIXMEs
for points where I'd either like you to check new text that I
added (in case it contains errors) or where I hope you can
provide answers to questions relating to details that may need
clarifying in the page.

I've appended the revised page at the foot of this mail. You can also
find the branch holding this page in Git at:
http://git.kernel.org/cgit/docs/man-pages/man-pages.git/log/?h=draft_seccomp

Notable changes from the previous draft:
* Several new error cases added under ERRORS
* New subsection on Seccomp-specific BPF details
* Add some detail in discussion of 'siginfo_t' fields
* Tweaked comments on BPF program in EXAMPLE section
* Added various FIXMEs

I also have one API quibble, regarding the name of the
SYS_SECCOMP constant; see below.

Feedback as inline comments to the below would be great!

Cheers,

Michael

.\" Copyright (C) 2014 Kees Cook <keescook@chromium.org>
.\" and Copyright (C) 2012 Will Drewry <wad@chromium.org>
.\" and Copyright (C) 2008, 2014 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.TH SECCOMP 2 2014-06-23 "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>

.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.

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),
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.

This operation is available only if the kernel is configured with
.BR CONFIG_SECCOMP
enabled.

The value of
.IR flags
must be 0, and
.IR args
must be NULL.

This operation is functionally identical to the call:

prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);
.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 .

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).

In order to use the
.BR SECCOMP_SET_MODE_FILTER
operation, either the caller must have the
.BR CAP_SYS_ADMIN
capability, 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:

prctl(PR_SET_NO_NEW_PRIVS, 1);

Otherwise, the
.BR SECCOMP_SET_MODE_FILTER
operation will fail and return
.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 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
.BR prctl (2)
or
.BR 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
.BR SECCOMP_SET_MODE_FILTER
operation is available only if the kernel is configured with
.BR CONFIG_SECCOMP_FILTER
enabled.

When
.IR flags
is 0, this operation is functionally identical to the call:

prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, args);

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.)

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.
.RE
.SS Filters
When adding filters via
.BR SECCOMP_SET_MODE_FILTER ,
.IR args
points to a filter program:

.in +4n
.nf
struct sock_fprog {
unsigned short len; /* Number of BPF instructions */
struct sock_filter *filter; /* Pointer to array of
BPF instructions */
};
.fi
.in

Each program must contain one or more BPF instructions:

.in +4n
.nf
struct sock_filter { /* Filter block */
__u16 code; /* Actual filter code */
__u8 jt; /* Jump true */
__u8 jf; /* Jump false */
__u32 k; /* Generic multiuse field */
};
.fi
.in

.\" FIXME I reworded/enhanced the following sentence. Is it okay?
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 buffer of the following form:

.in +4n
.nf
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 */
};
.fi
.in

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 )
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.

If multiple filters exist, they are all executed,
in reverse order of their addition to the filter tree
(i.e., the most recently installed filter is executed first).
The return value for the evaluation of a given system call is the first-seen
.BR 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:
.TP
.BR SECCOMP_RET_KILL
This value results in the process exiting immediately
without executing the system call.
The process terminates as though killed by a
.B SIGSYS
signal
.RI ( not
.BR SIGKILL ).
.TP
.BR SECCOMP_RET_TRAP
This value results in the kernel sending a
.BR SIGSYS
signal to the triggering process without executing the system call.
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
.\" FIXME Why is the constant thus named? All of the other 'si_code'
.\" constants are prefixed 'SI_'. Why the inconsistency?
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., 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 sensible.
.\" FIXME Regarding the preceding line, can you give an example(s)
.\" of "something sensible"? (Depending on the answer, maybe it
.\" might be useful to add some text on this point.)
.\"
.\" FIXME Please check:
.\" In an attempt to make the text clearer, I changed
.\" "replacing it with" to "setting the return value register to"
.\" Okay?
(The architecture dependency is because setting the return value register to
.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 .

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 .

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
.B "must not"
allow use of
.BR ptrace (2)\(emeven
of other
sandboxed processes\(emwithout extreme care;
.\" FIXME Below, I think it would be helpful to add some words after
.\" "to escape", as in "to escape [what?]" I suppose the wording
.\" would be something like "to escape the seccomp sandbox mechanism"
.\" but perhaps you have a better wording.
ptracers can use this mechanism to escape.)
.TP
.BR SECCOMP_RET_ALLOW
This value results in the system call being executed.
.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 EACCESS
The caller did not have the
.BR CAP_SYS_ADMIN
capability, 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
.IR flags
are invalid for the given
.IR operation .
.\" FIXME Please review the following
.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)" .
.\" FIXME Please review the following
.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.
.\" FIXME Please review the following
.TP
.BR EINVAL
.\" See stub kernel/seccomp.c::seccomp_set_mode_filter() in 3.18 sources
.I operation
specified
.BR SECCOMP_SET_MODE_FILTER ,
but the kernel was not built with
.B CONFIG_SECCOMP_FILTER
enabled.
.\" FIXME Please review the following
.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.
.BR EINVAL
.TP
.BR ENOMEM
Out of memory.
.\" FIXME Please review the following
.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 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
.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 ).
.\" FIXME Please review the following new subsection {{{
.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.
.\" FIXME What is the significance of the line
.\" ftest->code = BPF_LDX | BPF_W | BPF_ABS;
.\" in kernel/seccomp.c::seccomp_check_filter()?
.IP *
The
.B BPF_LEN
addressing mode modifier yields an immediate mode operand
whose value is the size of the
.IR seccomp_data
buffer.
.\" FIXME Any other seccomp-specific BPF details that should be added here?
.\"
.\" FIXME End of new subsection for review }}}
.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 execve (3)
(a library function that employs the
.BR 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:

.nf
.in +4n
$ \fBuname -m\fP
x86_64
$ \fBsyscall_nr() {
cat /usr/src/linux/arch/x86/syscalls/syscall_64.tbl | \\
awk '$2 != "x32" && $3 == "'$1'" { print $1 }'
}\fP
.in
.fi

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:

.nf
.in +4n
$ \fBerrno 99\fP
EADDRNOTAVAIL 99 Cannot assign requested address
.in
.fi

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:

.nf
.in +4n
$ \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
.in
.fi

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:

.nf
.in +4n
$ \fBsyscall_nr write\fP
1
$ \fB./a.out 1 0xC000003E 99 /bin/whoami\fP
.in
.fi

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:

.nf
.in +4n
$ \fBsyscall_nr preadv\fP
295
$ \fB./a.out 295 0xC000003E 99 /bin/whoami\fP
cecilia
.in
.fi
.SS Program source
.fi
.nf
#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>

static int
install_filter(int syscall_nr, int t_arch, int f_errno)
{
.\" FIXME In the BPF program below, you use '+' to build the instructions.
.\" However, most other BPF example code I see uses '|'. While I
.\" assume it's equivalent (i.e., the bit fields are nonoverlapping),
.\" was there a reason to use '+' rather than '|'? (To me, the
.\" latter is a little clearer in its intent.)
.\"
.\" FIXME I expanded comments [0], [1], [2], [3], [4] a little.
.\" Are they okay? */
.\"
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 4 instructions if architecture does not
match 't_arch' */
BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, t_arch, 0, 4),

/* [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] 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),

/* [4] 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)),

/* [5] Destination of system call number mismatch: allow other
system calls */
BPF_STMT(BPF_RET + BPF_K, SECCOMP_RET_ALLOW),

/* [6] 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);
}
.fi
.SH SEE ALSO
.BR prctl (2),
.BR ptrace (2),
.BR signal (7),
.BR socket (7)
.sp
The kernel source files
.IR Documentation/networking/filter.txt
and
.IR Documentation/prctl/seccomp_filter.txt .
.sp
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

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/


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