Re: [PATCH] net/p9/trans_fd.c: fix double list_del()

[Dominique Martinet <asmadeus@codewreck.org>]

Tomas Bortoli wrote on Mon, Jul 23, 2018:
> A double list_del(&req->req_list) is possible in p9_fd_cancel() as
> shown by Syzbot. To prevent it we have to ensure that we have the
> client->lock when deleting the list. Furthermore, we have to update
> the status of the request before releasing the lock, to prevent the
> race.

Nice, so no need to change the list_del to list_del_init!

I still have a nitpick on the last moved unlock, but it's mostly
aesthetic - the change looks much better to me now.

(Since that will require a v2 I'll be evil and go further than Yiwen
about the commit message: let it breathe a bit! :) I think a line break
before "furthermore" for example will make it easier to read)

> 
> Signed-off-by: Tomas Bortoli <tomasbortoli@gmail.com>
> Reported-by: syzbot+735d926e9d1317c3310c@syzkaller.appspotmail.com
> ---
>  net/9p/trans_fd.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
> index a64b01c56e30..370c6c69a05c 100644
> --- a/net/9p/trans_fd.c
> +++ b/net/9p/trans_fd.c
> @@ -199,15 +199,14 @@ static void p9_mux_poll_stop(struct p9_conn *m)
>  static void p9_conn_cancel(struct p9_conn *m, int err)
>  {
>  	struct p9_req_t *req, *rtmp;
> -	unsigned long flags;
>  	LIST_HEAD(cancel_list);
>  
>  	p9_debug(P9_DEBUG_ERROR, "mux %p err %d\n", m, err);
>  
> -	spin_lock_irqsave(&m->client->lock, flags);
> +	spin_lock(&m->client->lock);
>  
>  	if (m->err) {
> -		spin_unlock_irqrestore(&m->client->lock, flags);
> +		spin_unlock(&m->client->lock);
>  		return;
>  	}
>  
> @@ -219,7 +218,6 @@ static void p9_conn_cancel(struct p9_conn *m, int err)
>  	list_for_each_entry_safe(req, rtmp, &m->unsent_req_list, req_list) {
>  		list_move(&req->req_list, &cancel_list);
>  	}
> -	spin_unlock_irqrestore(&m->client->lock, flags);
>  
>  	list_for_each_entry_safe(req, rtmp, &cancel_list, req_list) {
>  		p9_debug(P9_DEBUG_ERROR, "call back req %p\n", req);
> @@ -228,6 +226,7 @@ static void p9_conn_cancel(struct p9_conn *m, int err)
>  			req->t_err = err;
>  		p9_client_cb(m->client, req, REQ_STATUS_ERROR);
>  	}
> +	spin_unlock(&m->client->lock);
>  }
>  
>  static __poll_t
> @@ -370,12 +369,12 @@ static void p9_read_work(struct work_struct *work)
>  		if (m->req->status != REQ_STATUS_ERROR)
>  			status = REQ_STATUS_RCVD;
>  		list_del(&m->req->req_list);
> -		spin_unlock(&m->client->lock);
>  		p9_client_cb(m->client, m->req, status);
>  		m->rc.sdata = NULL;
>  		m->rc.offset = 0;
>  		m->rc.capacity = 0;
>  		m->req = NULL;
> +		spin_unlock(&m->client->lock);

It took me a while to understand why you extended this lock despite
having just read the commit message, I'd suggest:
 - moving the spin_unlock to right after p9_client_cb (afterall that's
what we want, the m->rc and m->req don't need to be protected)
 - add a comment before p9_client_cb saying something like 'updates
req->status' or try to explain why it needs to be locked here but other
transports don't need such a lock (they're not dependant on req->status
like this)

-- 
Dominique