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

From: [email-masked] (Oleg Nesterov)
Date: Wed, 25 Jun 2014 18:52:09 +0200

On 06/25, Kees Cook wrote:
>
> On Wed, Jun 25, 2014 at 7:21 AM, Oleg Nesterov <[email-masked]> 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.