Re: [PATCH] seccomp.2: Add note about alarm(2) not being sufficient to limit runtime

[Michael Kerrisk (man-pages)]

Hello Jann,

On 03/12/2015 02:07 PM, Jann Horn wrote:
> On Wed, Mar 11, 2015 at 10:43:50PM +0100, Mikael Pettersson wrote:
>> Jann Horn writes:
>>  > Or should I throw this patch away and write a patch
>>  > for the prctl() manpage instead that documents that
>>  > being able to call sigreturn() implies being able to
>>  > effectively call sigprocmask(), at least on some
>>  > architectures like X86?
>>
>> Well, that is the semantics of sigreturn().  It is essentially
>> setcontext() [which includes the actions of sigprocmask()], but
>> with restrictions on parameter placement (at least on x86).
>>
>> You could introduce some setting to restrict that aspect for
>> seccomp processes, but you can't change this for normal processes
>> without breaking things.
> 
> Then I think it's probably better and easier to just document the existing
> behavior? If a new setting would have to be introduced and developers would
> need to be aware of that, it's probably easier to just tell everyone to use
> SIGKILL.
> 
> Does this manpage patch look good?

Patch applied, with Acks from Andy, Mikael, and Kees (I don't
usually get patches whose pedigree is that good. Thanks!)

I tweaked a few wordings. You can find the changes in Git [1]

Cheers,

Michael

[1] 
http://git.kernel.org/cgit/docs/man-pages/man-pages.git/commit/?id=65be1b46fb88e14f0af494ac6b53a2d6a63bb860

> ---
>  man2/seccomp.2 | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/man2/seccomp.2 b/man2/seccomp.2
> index 702ceb8..f762d07 100644
> --- a/man2/seccomp.2
> +++ b/man2/seccomp.2
> @@ -64,6 +64,31 @@ 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
> +.BR sigprocmask (2),
> +it can use
> +.BR sigreturn (2)
> +to block all signals apart from
> +.BR SIGKILL
> +and
> +.BR SIGSTOP .
> +Therefore, to reliably terminate it,
> +.BR SIGKILL
> +has to be used, meaning that e.g.
> +.BR alarm (2)
> +is not sufficient for restricting its runtime. Instead, use
> +.BR timer_create (2)
> +with
> +.BR SIGEV_SIGNAL
> +and
> +.BR sigev_signo
> +set to
> +.BR SIGKILL
> +or use
> +.BR setrlimit (2)
> +to set the hard limit for
> +.BR RLIMIT_CPU .
> +
>  This operation is available only if the kernel is configured with
>  .BR CONFIG_SECCOMP
>  enabled.
> 


-- 
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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