From: Mike Snitzer <snitzer@gmail.com>

NBD does not protect the nbd_device's socket from becoming NULL during
receives.

This closes a race with the NBD_CLEAR_SOCK ioctl (nbd-client -d) setting
the nbd_device's socket to NULL right before NBD calls sock_xmit.

Signed-off-by: Mike Snitzer <snitzer@gmail.com>
Cc: Paul Clements <paul.clements@steeleye.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 drivers/block/nbd.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff -puN drivers/block/nbd.c~nbd-prevent-sock_xmit-from-attempting-to-use-a-null-socket drivers/block/nbd.c
--- a/drivers/block/nbd.c~nbd-prevent-sock_xmit-from-attempting-to-use-a-null-socket
+++ a/drivers/block/nbd.c
@@ -153,6 +153,12 @@ static int sock_xmit(struct nbd_device *
 	struct kvec iov;
 	sigset_t blocked, oldset;
 
+	if (unlikely(!sock)) {
+		printk(KERN_ERR "%s: Attempted %s on closed socket in sock_xmit\n",
+		       lo->disk->disk_name, (send ? "send" : "recv"));
+		return -EINVAL;
+	}
+
 	/* Allow interception of SIGKILL only
 	 * Don't allow other signals to interrupt the transmission */
 	siginitsetinv(&blocked, sigmask(SIGKILL));
_