linux-kernel - Re: [PATCH] seccomp: dump core when using SECCOMP_RET

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Date:   Fri, 20 Jan 2017 12:37:11 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Mike Frysinger <vapier@...too.org>,
        James Morris <james.l.morris@...cle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>,
        Mike Frysinger <vapier@...omium.org>
Subject: Re: [PATCH] seccomp: dump core when using SECCOMP_RET_KILL

On Thu, Jan 19, 2017 at 8:28 PM, Mike Frysinger <vapier@...too.org> wrote:
> From: Mike Frysinger <vapier@...omium.org>
>
> The SECCOMP_RET_KILL mode is documented as immediately killing the
> process as if a SIGSYS had been sent and not caught (similar to a
> SIGKILL).  However, a SIGSYS is documented as triggering a coredump
> which does not happen today.
>
> This has the advantage of being able to more easily debug a process
> that fails a seccomp filter.  Today, most apps need to recompile and
> change their filter in order to get detailed info out, or manually run
> things through strace, or enable detailed kernel auditing.  Now we get
> coredumps that fit into existing system-wide crash reporting setups.
>
> From a security pov, this shouldn't be a problem.  Unhandled signals
> can already be sent externally which trigger a coredump independent of
> the status of the seccomp filter.  The act of dumping core itself does
> not cause change in execution of the program.
>
> URL: https://crbug.com/676357
> Signed-off-by: Mike Frysinger <vapier@...omium.org>
> Acked-by: Jorge Lucangeli Obes <jorgelo@...omium.org>

Yup, I think this is fine. The additional kernel code executed before
the do_exit() is relatively limited, and is equivalent to leaving
kill(self, SIGSEGV) exposed in a seccomp filter. Setting an RLIMIT is
also sufficient to block the core generation, so really paranoid
environments can still do that.

The forwarded ack stands:

Acked-by: Kees Cook <keescook@...omium.org>

James, can you add this to your tree?

-Kees

-- 
Kees Cook
Nexus Security