Messages in this thread | | | Date | Mon, 2 Sep 2019 18:20:36 +0200 | From | Peter Zijlstra <> | Subject | Re: [BUG] Use of probe_kernel_address() in task_rcu_dereference() without checking return value |
| |
On Mon, Sep 02, 2019 at 04:44:24PM +0200, Oleg Nesterov wrote:
> speaking of the users of task_rcu_dereference(), membarrier_global_expedited() > does > > rcu_read_lock(); > p = task_rcu_dereference(&cpu_rq(cpu)->curr); > if (p && p->mm && (atomic_read(&p->mm->membarrier_state) & > MEMBARRIER_STATE_GLOBAL_EXPEDITED)) { > > This asks for READ_ONCE, but this is minor. Why can't p->mm be freed? > > I guess it is fine to read the garbage from &p->mm->membarrier_state if we race > with the exiting task, but in theory this looks unsafe if CONFIG_DEBUG_PAGEALLOC. > > Another possible user of probe_slab_address() or I am totally confused?
You're quite right; that's busted.
Due to the lack of READ_ONCE() on p->mm, the above can in fact turn into a NULL deref when we hit do_exit() around exit_mm(). The first p->mm read is before and sees !NULL, the second is after and does observe NULL, and *bang*.
I suppose it wants to be something like:
mm = READ_ONCE(p->mm); if (mm && probe_address())
(I'm not sure _slab_ is a useful part of the name; it should work on kernel memory irrespective of the allocator)
If it got freed, that CPU already just did something that implies smp_mb() so we're good. So whatever garbage gets read, is fine. Either we do a superfluous IPI or not is immaterial.
| |