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

[Andy Lutomirski <luto@amacapital.net>]

On Thu, Mar 12, 2015 at 6:07 AM, Jann Horn <jann@thejh.net> 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?

Looks good to me.

FWIW, if we wanted to fix this in the kernel, I think it could be
easier to add SIG_KILL which would be just like SIG_DFL except always
fatal even if masked rather than coming up with complicated changes to
sigreturn.

--Andy

>
> ---
>  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.
> --
> 2.1.4
>



-- 
Andy Lutomirski
AMA Capital Management, LLC