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 [off-list ref] wrote:

quoted

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.