On Wed, 03 Aug 2022 06:38:36 +0530  Eric Biggers [off-list ref] wrote:

On Thu, Jul 28, 2022 at 09:21:21PM +0530, Siddh Raman Pant wrote:

quoted

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 [off-list ref]
    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.

quoted

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.

quoted

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