linux-kernel - Re: seccomp vs ptrace

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Message-ID: <CAGXu5jLwSFKeC=_mF53Dn6pUJ6mbp9UVfam+LvY2xRDth5S-DQ@mail.gmail.com>
Date:	Wed, 18 Mar 2015 14:38:48 -0700
From:	Kees Cook <keescook@...omium.org>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>, stgraber@...ntu.com
Subject: Re: seccomp vs ptrace

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.

Basically, the note is correct: ptrace really cannot be allowed in a
filter or it could be used to bypass the filter.

I am, of course, open to ideas on fixing this without introducing
loops. The use-case of having a ptrace monitor of a sandboxed process
intentionally changing syscalls isn't a use-case I think we need to
support. Monitors like that should just perform such calls themselves
from the unsandboxed half.

-Kees

-- 
Kees Cook
Chrome OS Security
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