----- On Sep 3, 2019, at 12:23 PM, Linus Torvalds [email protected] wrote:
> On Tue, Sep 3, 2019 at 9:00 AM Mathieu Desnoyers > <[email protected]> wrote: >> >> Due to the lack of READ_ONCE() on p->mm, this code 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, which triggers a null pointer dereference. > > This is horribly ugly, and I don't think it is necessary. > > The way to fix the problem you are addressing in patches 2-3 is to > move the MEMBARRIER_STATE_GLOBAL_EXPEDITED flag from the mm struct to > the task struct, and avoiding the whole issue with "mm may be released > at any point" that way. > > Now, your reaction will be "but lots of threads can share an 'mm', so > we can't do that", but that doesn't seem to be true. Looking at the > place that _sets_ this, you already handle the single-thread cases > specially, and the multiple threads has to do a "synchronize_rcu()". > You might as well either walk the current CPU's and set it in all > threads where the thread->mm matches the mm. And then you make the > scheduler set the bit on newly scheduled entities. > > NOTE! When you walk all current cpu's in > membarrier_register_global_expedited(), you only look at the > 'task->mm' _value_, you don't dereference it. And that's ok, because > 'task' itself is stable, it's just mm that can go away. > > Wouldn't that solve the issue much more cleanly? Indeed it would! I just sent a patch as RFC implementing your idea. Thanks! Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
