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

[Tomas Bortoli <tomasbortoli@gmail.com>]

On 07/23/2018 02:57 PM, Dominique Martinet wrote:
> 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)
> 

agree

>>
>> 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)

yes, better.

>  - 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)
> 

ok

thanks for the feedback