From: Josef Bacik <josef@toxicpanda.com>
Date: Mon, 25 Jan 2021 12:21:02 -0500
Subject: nbd: freeze the queue while we're adding connections
Patch-mainline: v5.11-rc6
Git-commit: b98e762e3d71e893b221f871825dc64694cfb258
References: bsc#1181504 CVE-2021-3348

When setting up a device, we can krealloc the config->socks array to add
new sockets to the configuration.  However if we happen to get a IO
request in at this point even though we aren't setup we could hit a UAF,
as we deref config->socks without any locking, assuming that the
configuration was setup already and that ->socks is safe to access it as
we have a reference on the configuration.

But there's nothing really preventing IO from occurring at this point of
the device setup, we don't want to incur the overhead of a lock to
access ->socks when it will never change while the device is running.
To fix this UAF scenario simply freeze the queue if we are adding
sockets.  This will protect us from this particular case without adding
any additional overhead for the normal running case.

Cc: stable@vger.kernel.org
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Acked-by: Daniel Wagner <dwagner@suse.de>
---
 drivers/block/nbd.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -871,6 +871,12 @@ static int nbd_add_socket(struct nbd_dev
 	if (!sock)
 		return err;
 
+	/*
+	 * We need to make sure we don't get any errant requests while we're
+	 * reallocating the ->socks array.
+	 */
+	blk_mq_freeze_queue(nbd->disk->queue);
+
 	if (!netlink && !nbd->task_setup &&
 	    !test_bit(NBD_BOUND, &config->runtime_flags))
 		nbd->task_setup = current;
@@ -909,10 +915,12 @@ static int nbd_add_socket(struct nbd_dev
 	nsock->cookie = 0;
 	socks[config->num_connections++] = nsock;
 	atomic_inc(&config->live_connections);
+	blk_mq_unfreeze_queue(nbd->disk->queue);
 
 	return 0;
 
 put_socket:
+	blk_mq_unfreeze_queue(nbd->disk->queue);
 	sockfd_put(sock);
 	return err;
 }