nbd: prevent sock_xmit from attempting to use a NULL socket

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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index b53fdb0a282c..60cc54368b66 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -153,6 +153,12 @@ static int sock_xmit(struct nbd_device *lo, int send, void *buf, int size,
         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));