Re: [PATCH] nbd: pass nbd_sock to nbd_read_reply() instead of index

[Ming Lei]

On Mon, Sep 11, 2023 at 10:33:08AM +0800, linan666@xxxxxxxxxxxxxxx wrote:
> From: Li Nan <linan122@xxxxxxxxxx>
> 
> If a socket is processing ioctl 'NBD_SET_SOCK', config->socks might be
> krealloc in nbd_add_socket(), and a garbage request is received now, a UAF
> may occurs.
> 
>   T1
>   nbd_ioctl
>    __nbd_ioctl
>     nbd_add_socket
>      blk_mq_freeze_queue
> 				T2
>   				recv_work
>   				 nbd_read_reply
>   				  sock_xmit
>      krealloc config->socks
> 				   def config->socks
> 
> Pass nbd_sock to nbd_read_reply(). And introduce a new function
> sock_xmit_recv(), which differs from sock_xmit only in the way it get
> socket.
> 

I am wondering why not grab queue usage counter before calling nbd_read_reply()
for avoiding such issue, something like the following change:

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index df1cd0f718b8..09215b605b12 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -837,9 +837,6 @@ static void recv_work(struct work_struct *work)
 	while (1) {
 		struct nbd_reply reply;
 
-		if (nbd_read_reply(nbd, args->index, &reply))
-			break;
-
 		/*
 		 * Grab .q_usage_counter so request pool won't go away, then no
 		 * request use-after-free is possible during nbd_handle_reply().
@@ -852,6 +849,9 @@ static void recv_work(struct work_struct *work)
 			break;
 		}
 
+		if (nbd_read_reply(nbd, args->index, &reply))
+			break;
+
 		cmd = nbd_handle_reply(nbd, args->index, &reply);
 		if (IS_ERR(cmd)) {
 			percpu_ref_put(&q->q_usage_counter);

Thanks,
Ming