[PATCH] kernel/watch_queue: Make pipe NULL while clearing watch_queue

[Greg KH]

On Sat, Jul 23, 2022 at 07:24:47PM +0530, Siddh Raman Pant via Linux-kernel-mentees 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.
> 
> Bug report: https://syzkaller.appspot.com/bug?id=1870dd7791ba05f2ea7f47f7cbdde701173973fc
> Reported-and-tested-by: syzbot+c70d87ac1d001f29a058 at syzkaller.appspotmail.com
> 
> Signed-off-by: Siddh Raman Pant <code at siddh.me>
> ---
>  kernel/watch_queue.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
> index bb9962b33f95..bab9e09c74cf 100644
> --- a/kernel/watch_queue.c
> +++ b/kernel/watch_queue.c
> @@ -637,8 +637,17 @@ void watch_queue_clear(struct watch_queue *wqueue)
>  		spin_lock_bh(&wqueue->lock);
>  	}
>  
> -	spin_unlock_bh(&wqueue->lock);
>  	rcu_read_unlock();

Also you now have a spinlock held when calling rcu_read_unlock(), are
you sure that's ok?