Re: [PATCH] net/9p/trans_fd.c: fix race by holding the lock

[Dominique Martinet <asmadeus@codewreck.org>]

Tomas Bortoli wrote on Mon, Jul 23, 2018:
> It may be possible to run p9_fd_cancel() with a deleted req->req_list
> and incur in a double del. To fix hold the client->lock while changing
> the status, so the other threads will be synchronized.
> 
> Signed-off-by: Tomas Bortoli <tomasbortoli@gmail.com>
> Reported-by: syzbot+735d926e9d1317c3310c@syzkaller.appspotmail.com

OK, taking this one.

> ---
>  net/9p/trans_fd.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
> index 588bf88c3305..f9f96d50d96d 100644
> --- a/net/9p/trans_fd.c
> +++ b/net/9p/trans_fd.c
> @@ -197,15 +197,14 @@ 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;
>  	}
> 
> @@ -217,7 +216,6 @@ 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);
> @@ -226,6 +224,7 @@ 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
> @@ -373,8 +372,9 @@ 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);
> +		/* update req->status while holding client->lock  */
>  		p9_client_cb(m->client, m->req, status);
> +		spin_unlock(&m->client->lock);
>  		m->rc.sdata = NULL;
>  		m->rc.offset = 0;
>  		m->rc.capacity = 0;