Messages in this thread | | | Date | Wed, 25 Jun 2014 16:21:21 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC |
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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 the 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?
Oleg.
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