linux-kernel - Re: [PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG

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Message-ID: <20140625165209.GA14720@redhat.com>
Date:	Wed, 25 Jun 2014 18:52:09 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Will Drewry <wad@...omium.org>,
	Julien Tinnes <jln@...omium.org>,
	David Drysdale <drysdale@...gle.com>,
	Linux API <linux-api@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, linux-mips@...ux-mips.org,
	linux-arch <linux-arch@...r.kernel.org>,
	linux-security-module <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC

On 06/25, Kees Cook wrote:
>
> On Wed, Jun 25, 2014 at 7:21 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > But. Doesn't this change add a new security hole?
> >
> > Obviously, we should not allow to install a filter and then (say) exec
> > a suid binary, that is why we have no_new_privs/LSM_UNSAFE_NO_NEW_PRIVS.
> >
> > But what if "thread->seccomp.filter = caller->seccomp.filter" races with
> > any user of task_no_new_privs() ? Say, suppose this thread has already
> > passed check_unsafe_exec/etc and it is going to exec the suid binary?
>
> Oh, ew. Yeah. It looks like there's a cred lock to be held to combat this?

Yes, cred_guard_mutex looks like an obvious choice... Hmm, but somehow
initially I thought that the fix won't be simple. Not sure why.

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?

But I was also worried that task_no_new_privs(current) is no longer stable
inside the syscall paths, perhaps this is what you meant? However I do not
see something bad here... And this has nothing to do with the race above.

Also. Even ignoring no_new_privs, SECCOMP_FILTER_FLAG_TSYNC is not atomic
and we can do nothing with this fact (unless it try to freeze the thread
group somehow), perhaps it makes sense to document this somehow.

I mean, suppose you want to ensure write-to-file is not possible, so you
do seccomp(SECCOMP_FILTER_FLAG_TSYNC, nack_write_to_file_filter). You can't
assume that this has effect right after seccomp() returns, this can obviously
race with a sub-thread which has already entered sys_write().

Once again, I am not arguing, just I think it makes sense to at least mention
the limitations during the discussion.

Oleg.

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