On Wed, Jun 25, 2014 at 10:24 AM, Oleg Nesterov <firstname.lastname@example.org> wrote:
> On 06/25, Kees Cook wrote:
>> On Wed, Jun 25, 2014 at 9:52 AM, Oleg Nesterov <email@example.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
> thared has already passed (say) check_unsafe_exec().
> However, do_execve() takes cred_guard_mutex at the start in
> and drops it in install_exec_creds(), so it should solve the problem?
If you rely on this, then please fix this comment in fs/exec.c:
* determine how safe it is to execute the proposed program
* - the caller must hold ->cred_guard_mutex to protect against
static void check_unsafe_exec(struct linux_binprm *bprm)
It sounds like cred_guard_mutex is there for exactly this reason :)