linux-kernel - Re: seccomp vs ptrace

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Date:	Wed, 18 Mar 2015 14:42:02 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Kees Cook <keescook@...omium.org>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Stephane Graber <stgraber@...ntu.com>
Subject: Re: seccomp vs ptrace

On Wed, Mar 18, 2015 at 2:38 PM, Kees Cook <keescook@...omium.org> wrote:
> On Wed, Mar 18, 2015 at 2:30 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
>> Hi,
>>
>> I'm writing to ask about
>>
>>         The seccomp check will not be run again after the tracer is
>>         notified.  (This means that seccomp-based sandboxes MUST NOT
>>         allow use of ptrace, even of other sandboxed processes, without
>>         extreme care; ptracers can use this mechanism to escape.)
>>
>> This basically means that seccomp cannot be safely used with for instance
>> an upstart based container.  I've been told that Andy was working on
>> changing the order so that ptrace checks would be done before seccomp.
>> Is there any update on that?  Is it likely to happen?  Scrapped?
>
> There are two problems, as I see it:
>
> 1) seccomp filtering happens first, so any following ptrace actions
> could change the syscall that actually happens (e.g. a filter allows
> clone and ptrace, meaning it could start a child, ptrace it, issue an
> allowed syscall, catch it, and change it to a disallowed syscall:
> escape from sandbox).
>
> 2) even if ptrace was moved ahead of seccomp, a sandboxed process as
> above and also access to add more filters (via seccomp or prctl
> syscalls) could use SECCOMP_RET_TRACE, to catch the syscall at the end
> of the seccomp checks, which would allow the same as above.

Ouch!

Arguably we messed up by making SECCOMP_RET_TRACE have higher
precedence than ERRNO and TRAP.  We could add new ERRNO and TRAP
actions that have high precedence or a new flag that promotes them in
the filter being applied.

--Andy
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