On Wed, Jun 25, 2014 at 7:21 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 06/24, Kees Cook wrote:
>>
>> +static void seccomp_sync_threads(void)
>> +{
>> + struct task_struct *thread, *caller;
>> +
>> + BUG_ON(!spin_is_locked(¤t->sighand->siglock));
>> +
>> + /* Synchronize all threads. */
>> + caller = current;
>> + for_each_thread(caller, thread) {
>> + /* Get a task reference for the new leaf node. */
>> + get_seccomp_filter(caller);
>> + /*
>> + * Drop the task reference to the shared ancestor since
>> + * current's path will hold a reference. (This also
>> + * allows a put before the assignment.)
>> + */
>> + put_seccomp_filter(thread);
>> + thread->seccomp.filter = caller->seccomp.filter;
>> + /* Opt the other thread into seccomp if needed.
>> + * As threads are considered to be trust-realm
>> + * equivalent (see ptrace_may_access), it is safe to
>> + * allow one thread to transition the other.
>> + */
>> + if (thread->seccomp.mode == SECCOMP_MODE_DISABLED) {
>> + /*
>> + * Don't let an unprivileged task work around
>> + * the no_new_privs restriction by creating
>> + * a thread that sets it up, enters seccomp,
>> + * then dies.
>> + */
>> + if (task_no_new_privs(caller))
>> + task_set_no_new_privs(thread);
>> +
>> + seccomp_assign_mode(thread, SECCOMP_MODE_FILTER);
>> + }
>> + }
>> +}
>
> OK, personally I think this all make sense. I even think that perhaps
> SECCOMP_FILTER_FLAG_TSYNC should allow filter == NULL, a thread might
> want to "sync" without adding the new filter, but this is minor/offtopic.
>
> 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?
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?
-Kees
--
Kees Cook
Chrome OS Security
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