Message-ID: <545E0B06.6030706@gmail.com>
Date: Sat, 08 Nov 2014 13:22:30 +0100
From: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
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To: Kees Cook <keescook@chromium.org>
CC: mtk.manpages@gmail.com,
        "linux-man@vger.kernel.org" <linux-man@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Linux API <linux-api@vger.kernel.org>,
        Daniel Borkmann <dborkman@redhat.com>
Subject: Edited seccomp.2 man page for review
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Sender: linux-kernel-owner@vger.kernel.org

Hi Kees, (and all),

Thanks for the seccomp.2 draft man page that you provided a few 
weeks ago (https://lkml.org/lkml/2014/9/25/685), and my apologies 
for the slow follow-up. 

I have done some substantial editing of the page. Therefore, could
you please carefully read the revised version below, in case I have
somewhere injected errors.

In addition, I've added a number of FIXMEs to the page source. Could
you please review these.

I've also added long piece to the example section, describing the 
program and demonstrating its use. Again, I'd appreciate it if you
could check that over.

One other question about these man-pages changes: should we add
a note in prctl(2) to say that seccomp(2) is preferred over
PR_SET_SECCOMP for new code?

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

Feedback either as inline comments to the below, or as a patch based on
the Git branch, 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>
.\"
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.\" manual page may be incorrect or out-of-date.  The author(s) assume no
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.\" have taken the same level of care in the production of this manual,
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.\"
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.\"
.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>
.\" FIXME Is sys/ptrace.h really required? It is not used in
.\"       the example program below.
.B #include <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.
.\" FIXME: This page various uses the terms "process', "thread" and "task".
.\" Probably only one of these (not "task"!) should be used in all
.\" cases. I suspect it should be "thread".

Currently, Linux supports the following
.IR operation
values:
.TP
.BR SECCOMP_SET_MODE_STRICT
The only system calls that the 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 EACCESS
in
.IR errno .

.\" FIXME I (mtk) reworded the following paragraph substantially.
.\" Please check it.
If
.BR fork (2)
or
.BR clone (2)
is allowed by the filter, any child processes will be constrained to
the same filters and system calls as the parent.
If
.BR execve (2)
is allowed by the filter,
the filters and constraints on permitted system calls are preserved across an
.BR execve (2).

.\" FIXME I (mtk) reworded the following paragraph substantially.
.\" Please check it.
In order to use the
.BR SECCOMP_SET_MODE_FILTER
operation, either the caller must have the
.BR CAP_SYS_ADMIN
capability or the call must be preceded by the 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 filter programs cannot be applied to child
.\" FIXME What does "installed" in the following line mean?
processes with greater privileges than the process that installed them.

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 process.

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.
.\" FIXME Nowhere in this page is the term "filter tree" defined.
.\" There should be a definition somewhere.
.\" Is it: "the set of filters attached to a thread"?
If any thread cannot do this,
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 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
.SH 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;
};
.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

When executing the instructions, the BPF program executes over the
system call information made available via:

.in +4n
.nf
struct seccomp_data {
    int nr;                     /* system call number */
    __u32 arch;                 /* AUDIT_ARCH_* value */
    __u64 instruction_pointer;  /* CPU instruction pointer */
    __u64 args[6];              /* up to 6 system call arguments */
};
.fi
.in

.\" FIXME I find the next piece a little hard to understand, so,
.\"       some questions:
.\"       * If there are multiple filters, in what order are they executed?
.\"         (The man page should probably detail the answer to this question.)
.\"       * If there are multiple filters, are they all always executed?
.\"         I assume not, but the notion that 
.\"             "the return value for the evaluation of a given system call
.\"              will always use the value with the highest precedence"
.\"         implies that even that if one filter generates (say)
.\"         SECCOMP_RET_ERRNO, then further filters may still be executed,
.\"         including one that generates (say) the "higher priority"
.\"         SECCOMP_RET_KILL condition.
.\"       Can you clarify the above?
A seccomp filter returns one of the values listed below.
If multiple filters exist,
the return value for the evaluation of a given system call
will always use the value with the highest precedence.
(For example,
.BR SECCOMP_RET_KILL
will always take precedence.)

In decreasing order order of precedence,
the values that may be returned by a seccomp filter are:
.TP
.BR SECCOMP_RET_KILL
Results in the task exiting immediately without executing the system call.
The task terminates as though killed by a
.B SIGSYS
signal
.RI ( not
.BR SIGKILL ).
.TP
.BR SECCOMP_RET_TRAP
Results in the kernel sending a
.BR SIGSYS
signal to the triggering task without executing the system call.
.IR siginfo\->si_call_addr
will show the address of the system call instruction, and
.IR siginfo\->si_syscall
and
.IR siginfo\->si_arch
will indicate which system call was attempted.
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.
(The architecture dependency is because replacing it with
.BR ENOSYS
could overwrite some useful information.)

.\" FIXME The following sentence is the first time that SECCOMP_RET_DATA
.\"       is mentioned. SECCOMP_RET_DATA needs to be described in this
.\"       man page.
The
.BR SECCOMP_RET_DATA
portion of the return value will be passed as
.IR si_errno .

.BR SIGSYS
triggered by seccomp will have the value
.BR SYS_SECCOMP
in the
.IR si_code
field.
.TP
.BR SECCOMP_RET_ERRNO
.\" FIXME What does "the return value" refer to in the next sentence?
.\"       It is not obvious to me.
Results in the lower 16-bits of the return value being passed
to user space as the
.IR errno
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.
.\" FIXME I (mtk) reworded the following sentence substantially.
.\" Please check it.
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 BPF program 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;
ptracers can use this mechanism to escape.)
.TP
.BR SECCOMP_RET_ALLOW
Results in the system call being executed.
.PP
If multiple filters exist, the return value for the evaluation of a
given system call will always use the highest precedent value.

.\" FIXME The following sentence is the first time that SECCOMP_RET_ACTION
.\"       is mentioned. SECCOMP_RET_ACTION needs to be described in this
.\"       man page.
Precedence is determined using only the
.BR SECCOMP_RET_ACTION
mask.
When multiple filters return values of the same precedence,
only the
.BR SECCOMP_RET_DATA
from the most recently installed filter will be returned.
.SH RETURN VALUE
On success,
.BR seccomp ()
returns 0.
On error, if
.BR SECCOMP_FILTER_FLAG_TSYNC
was used,
the return value is the thread ID that caused the synchronization failure.
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 required to be a valid address.
.TP
.BR EINVAL
.IR operation
is unknown; or
.IR flags
are invalid for the given
.IR operation
.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 ).
.SH EXAMPLE
.\" FIXME Please carefully review the following new piece that
.\"       demonstrates the use of your example program.
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 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, int arch, int error)
{
    struct sock_filter filter[] = {
        /* [0] Load architecture */
        BPF_STMT(BPF_LD + BPF_W + BPF_ABS,
                 (offsetof(struct seccomp_data, arch))),

        /* [1] Jump forward 4 instructions on architecture mismatch */
        BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, arch, 0, 4),

        /* [2] Load system call number */
        BPF_STMT(BPF_LD + BPF_W + BPF_ABS,
                 (offsetof(struct seccomp_data, nr))),

        /* [3] Jump forward 1 instruction on system call number
               mismatch */
        BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, syscall, 0, 1),

        /* [4] Matching architecture and system call: return
               specific errno */
        BPF_STMT(BPF_RET + BPF_K,
                 SECCOMP_RET_ERRNO | (error & 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:\\n"
                "refuse <syscall_nr> <arch> <errno> <prog> [<args>]\\n"
                "Hint:  AUDIT_ARCH_I386: 0x%X\\n"
                "       AUDIT_ARCH_X86_64: 0x%X\\n"
                "\\n", 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
.\" FIXME: Is the following the best source of info on the BPF language?
The kernel source file
.IR Documentation/networking/filter.txt .

-- 
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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