Messages in this thread | | | Date | Wed, 03 Aug 2022 09:26:04 +0530 | From | Siddh Raman Pant <> | Subject | Re: [PATCH v3] kernel/watch_queue: Make pipe NULL while clearing watch_queue |
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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.
> > > > Bug report: https://syzkaller.appspot.com/bug?id=1870dd7791ba05f2ea7f47f7cbdde701173973fc > > Reported-and-tested-by: syzbot+c70d87ac1d001f29a058@syzkaller.appspotmail.com > > If this actually does fix something, then it's mixing Fixes and Cc stable tags.
Noted.
> > diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c > > index bb9962b33f95..617425e34252 100644 > > --- a/kernel/watch_queue.c > > +++ b/kernel/watch_queue.c > > @@ -99,7 +99,7 @@ static bool post_one_notification(struct watch_queue *wqueue, > > struct watch_notification *n) > > { > > void *p; > > - struct pipe_inode_info *pipe = wqueue->pipe; > > + struct pipe_inode_info *pipe = READ_ONCE(wqueue->pipe); > > struct pipe_buffer *buf; > > struct page *page; > > unsigned int head, tail, mask, note, offset, len; > > @@ -637,6 +637,12 @@ void watch_queue_clear(struct watch_queue *wqueue) > > spin_lock_bh(&wqueue->lock); > > } > > > > + /* Clearing the watch queue, so we should clean the associated pipe. */ > > + if (wqueue->pipe) { > > + wqueue->pipe->watch_queue = NULL; > > + wqueue->pipe = NULL; > > + } > > + > > spin_unlock_bh(&wqueue->lock); > > rcu_read_unlock(); > > } > > And this is clearly the wrong fix anyway, since it makes the call to > put_watch_queue() in free_pipe_info() never be executed. So AFAICT, this patch > introduces a memory leak, and doesn't actually fix anything... > > - Eric >
Sorry for overlooking that. wqueue->pipe->watch_queue = NULL; is unnecessary, but wqueue->pipe = NULL; is needed.
I will send a final v4 once all issues are resolved in this thread.
Thanks, Siddh
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