Messages in this thread | | | Date | Tue, 2 Aug 2022 21:12:28 -0700 | From | Eric Biggers <> | Subject | Re: [PATCH v3] kernel/watch_queue: Make pipe NULL while clearing watch_queue |
| |
On Wed, Aug 03, 2022 at 09:26:04AM +0530, Siddh Raman Pant wrote: > On Wed, 03 Aug 2022 06:38:36 +0530 Eric Biggers <ebiggers@kernel.org> wrote: > > On Thu, Jul 28, 2022 at 09:21:21PM +0530, Siddh Raman Pant wrote: > > > If not done, a reference to a freed pipe remains in the watch_queue, > > > as this function is called before freeing a pipe in free_pipe_info() > > > (see line 834 of fs/pipe.c). > > > > > > This causes a UAF when post_one_notification() tries to access the pipe > > > on a key update, which is reported by syzbot. > > > > > > We also need to use READ_ONCE() in post_one_notification() to prevent the > > > compiler from optimising and loading a non-NULL value from wqueue->pipe. > > > > Didn't this already get fixed by the following commit? > > > > commit 353f7988dd8413c47718f7ca79c030b6fb62cfe5 > > Author: Linus Torvalds <torvalds@linux-foundation.org> > > Date: Tue Jul 19 11:09:01 2022 -0700 > > > > watchqueue: make sure to serialize 'wqueue->defunct' properly > > > > With that, post_one_notification() only runs while the watch_queue is locked and > > not "defunct". So it's guaranteed that the pipe still exists. Any concurrent > > free_pipe_info() waits for the watch_queue to be unlocked in watch_queue_clear() > > before proceeding to free the pipe. So where is there still a bug? > > It doesn't fix the dangling pointer to the freed pipe in the watch_queue, which > had caused this crash. >
Under what circumstances is the pipe pointer still being dereferenced after the pipe has been freed? I don't see how it can be; see my explanation above.
- Eric
| |