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Re: [PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC

To: Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC
From: Oleg Nesterov <oleg@redhat.com>
Date: Wed, 25 Jun 2014 19:24:10 +0200
Cc: LKML <linux-kernel@vger.kernel.org>, Andy Lutomirski <luto@amacapital.net>, "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>, Alexei Starovoitov <ast@plumgrid.com>, Andrew Morton <akpm@linux-foundation.org>, Daniel Borkmann <dborkman@redhat.com>, Will Drewry <wad@chromium.org>, Julien Tinnes <jln@chromium.org>, David Drysdale <drysdale@google.com>, Linux API <linux-api@vger.kernel.org>, "x86@kernel.org" <x86@kernel.org>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, linux-mips@linux-mips.org, linux-arch <linux-arch@vger.kernel.org>, linux-security-module <linux-security-module@vger.kernel.org>
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On 06/25, Kees Cook wrote:
>
> On Wed, Jun 25, 2014 at 9:52 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > Yes, at least this should close the race with suid-exec. And there are no
> > other users. Except apparmor, and I hope you will check it because I simply
> > do not know what it does ;)
> >
> >> I wonder if changes to nnp need to "flushed" during syscall entry
> >> instead of getting updated externally/asynchronously? That way it
> >> won't be out of sync with the seccomp mode/filters.
> >>
> >> Perhaps secure computing needs to check some (maybe seccomp-only)
> >> atomic flags and flip on the "real" nnp if found?
> >
> > Not sure I understand you, could you clarify?
>
> Instead of having TSYNC change the nnp bit, it can set a new flag, say:
>
>     task->seccomp.flags |= SECCOMP_NEEDS_NNP;
>
> This would be set along with seccomp.mode, seccomp.filter, and
> TIF_SECCOMP. Then, during the next secure_computing() call that thread
> makes, it would check the flag:
>
>     if (task->seccomp.flags & SECCOMP_NEEDS_NNP)
>         task->nnp = 1;
>
> This means that nnp couldn't change in the middle of a running syscall.

Aha, so you were worried about the same thing. Not sure we need this,
but at least I understand you and...

> Hmmm. Perhaps this doesn't solve anything, though? Perhaps my proposal
> above would actually make things worse, since now we'd have a thread
> with seccomp set up, and no nnp. If it was in the middle of exec,
> we're still causing a problem.

Yes ;)

> I think we'd also need a way to either delay the seccomp changes, or
> to notice this condition during exec. Bleh.

Hmm. confused again,

> What actually happens with a multi-threaded process calls exec? I
> assume all the other threads are destroyed?

Yes. But this is the point-of-no-return, de_thread() is called after the execing
thared has already passed (say) check_unsafe_exec().

However, do_execve() takes cred_guard_mutex at the start in prepare_bprm_creds()
and drops it in install_exec_creds(), so it should solve the problem?

Oleg.


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