Re: [PATCH v3] kernel/watch_queue: Make pipe NULL while clearing watch_queue
From: Eric Biggers <ebiggers@kernel.org>
Date: 2022-08-03 01:08:47
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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 [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?
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.
quoted hunk ↗ jump to hunk
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