linux-kernel - Re: [RESEND][PATCH] prctl: update seccomp sections for mode 2 (BPF)

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Date:	Mon, 22 Oct 2012 09:34:46 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	linux-man@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, wad@...omium.org,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RESEND][PATCH] prctl: update seccomp sections for mode 2 (BPF)

Kees,

A couple of questions about SECCOMP_MODE_FILTER.

I added some words that the arg3 is a pointer to 'struct fprog'. Can
you confirmn that's correct?

If the CONFIG_SECCOMP_FILTER permits fork(), is the seccomp setting
inherited across fork()? Similar question for execve().

Thanks,

Michael

On Mon, Oct 22, 2012 at 6:56 AM, Michael Kerrisk (man-pages)
<mtk.manpages@...il.com> wrote:
> Hello Kees,
>
> On Fri, Sep 21, 2012 at 1:37 AM, Kees Cook <keescook@...omium.org> wrote:
>> This adds a short summary of the arguments used for "mode 2" (BPF)
>> seccomp.
>
> Thanks very much. Applied.
>
> Cheers,
>
> Michael
>
>
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> ---
>>  man2/prctl.2 |   40 ++++++++++++++++++++++++++++++----------
>>  1 files changed, 30 insertions(+), 10 deletions(-)
>>
>> diff --git a/man2/prctl.2 b/man2/prctl.2
>> index eb53aa8..a44aa32 100644
>> --- a/man2/prctl.2
>> +++ b/man2/prctl.2
>> @@ -41,11 +41,12 @@
>>  .\" 2012-04 Cyrill Gorcunov, Document PR_SET_MM
>>  .\" 2012-04-25 Michael Kerrisk, Document PR_TASK_PERF_EVENTS_DISABLE and
>>  .\"                            PR_TASK_PERF_EVENTS_ENABLE
>> +.\" 2012-09-20 Kees Cook, update PR_SET_SECCOMP for mode 2
>>  .\"
>>  .\" FIXME: Document PR_SET_TIMERSLACK and PR_GET_TIMERSLACK (new in 2.6.28)
>>  .\"            commit 6976675d94042fbd446231d1bd8b7de71a980ada
>>  .\"
>> -.TH PRCTL 2 2012-04-23 "Linux" "Linux Programmer's Manual"
>> +.TH PRCTL 2 2012-09-20 "Linux" "Linux Programmer's Manual"
>>  .SH NAME
>>  prctl \- operations on a process
>>  .SH SYNOPSIS
>> @@ -235,11 +236,17 @@ in the location pointed to by
>>  .\" See http://thread.gmane.org/gmane.linux.kernel/542632
>>  .\" [PATCH 0 of 2] seccomp updates
>>  .\" andrea@...share.com
>> -Set the secure computing mode for the calling thread.
>> -In the current implementation,
>> +Set the secure computing mode for the calling thread, to limit
>> +the available syscalls.
>> +The seccomp mode is selected via
>> +.IR arg2 .
>> +(The seccomp constants are defined in
>> +.IR <linux/seccomp.h> .)
>> +
>> +With
>>  .IR arg2
>> -must be 1.
>> -After the secure computing mode has been set to 1,
>> +set to
>> +.BR SECCOMP_MODE_STRICT
>>  the only system calls that the thread is permitted to make are
>>  .BR read (2),
>>  .BR write (2),
>> @@ -249,22 +256,35 @@ and
>>  Other system calls result in the delivery of a
>>  .BR SIGKILL
>>  signal.
>> -Secure computing mode is useful for number-crunching applications
>> +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 only available
>>  if the kernel is configured with CONFIG_SECCOMP enabled.
>> +
>> +With
>> +.IR arg2
>> +set to
>> +.BR SECCOMP_MODE_FILTER " (since Linux 3.5)"
>> +the system calls allowed are
>> +defined by a pointer passed in
>> +.IR arg3
>> +to a Berkeley Packet Filter, which can be designed to filter
>> +arbitrary syscalls and syscall arguments.
>> +This mode is only available if the kernel is configured with
>> +CONFIG_SECCOMP_FILTER enabled.
>> +
>>  .TP
>>  .BR PR_GET_SECCOMP " (since Linux 2.6.23)"
>>  Return the secure computing mode of the calling thread.
>> -Not very useful for the current implementation (mode equals 1),
>> -but may be useful for other possible future modes:
>> -if the caller is not in secure computing mode, this operation returns 0;
>> -if the caller is in secure computing mode, then the
>> +If the caller is not in secure computing mode, this operation returns 0;
>> +if the caller is in strict secure computing mode, then the
>>  .BR prctl ()
>>  call will cause a
>>  .B SIGKILL
>>  signal to be sent to the process.
>> +If the caller is in filter mode, and this syscall is allowed by the
>> +seccomp filters, it returns 2.
>>  This operation is only available
>>  if the kernel is configured with CONFIG_SECCOMP enabled.
>>  .TP
>> --
>> 1.7.0.4
>>
>> --
>> Kees Cook
>> Chrome OS Security
>
>
>
> --
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Author of "The Linux Programming Interface"; http://man7.org/tlpi/



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface"; http://man7.org/tlpi/
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