linux-kernel - Re: seccomp vs ptrace

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Date:	Wed, 18 Mar 2015 14:44:56 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Andy Lutomirski <luto@...capital.net>
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 2:42 PM, Andy Lutomirski <luto@...capital.net> wrote:
> 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.

Nope, RET_TRACE is lower. KILL, TRAP, ERRNO, TRACE, ALLOW. Still
doesn't help the above cases, but we can't override a blocked syscall
just with a new filter. You'd still have to do the ptrace dance with
an allowed syscall.

-Kees

-- 
Kees Cook
Chrome OS Security
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