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

[Kees Cook]

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(&current->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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