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

From: Greg KH <gregkh@linuxfoundation.org>
To: Siddh Raman Pant <code@siddh.me>
Cc: David Howells <dhowells@redhat.com>,
	Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
	Eric Dumazet <edumazet@google.com>,
	"Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
	linux-security-modules <linux-security-module@vger.kernel.org>,
	linux-kernel-mentees 
	<linux-kernel-mentees@lists.linuxfoundation.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	syzbot+c70d87ac1d001f29a058@syzkaller.appspotmail.com
Subject: Re: [PATCH] kernel/watch_queue: Make pipe NULL while clearing watch_queue
Date: Sat, 23 Jul 2022 16:04:17 +0200	[thread overview]
Message-ID: <Ytv/4Tljvlt0PJ2r@kroah.com> (raw)
In-Reply-To: <20220723135447.199557-1-code@siddh.me>

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@syzkaller.appspotmail.com
> 
> Signed-off-by: Siddh Raman Pant <code@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?