[PATCH v4 3/7] nbd: remove queue freeze in nbd_add_socket

[Yang Erkun <yangerkun@huawei.com>]

The function nbd_add_socket use krealloc to expand the config->socks
array when adding new sockets. If I/O operations concurrently, this can
lead to UAF when accessing config->socks. Commit b98e762e3d71 ("nbd:
freeze the queue while we're adding connections") addressed this by
freezing the request_queue during the addtion. However, freezing and
unfreezing for multiprocess environments introduces noticeable latency,
espically there can be multi call for nbd_add_socket when one nbd setup.

Actually, after previous commit, nbd_add_socket cannot be called
after nbd_start_device, causing the device's capabilities to remain 0
and write cache disabled. This will make sure no I/O can happened
simultaneously(nbd accept I/O from bio):

 - read/write bios with sectors are rejected by bio_check_eod in
   submit_bio_noacct
 - flush-only bio for write cache disabled device will directly return 0
   in submit_bio_noacct (ps: we cannot enable write_cache for nbd device
   since without BLK_FEAT_WRITE_CACHE)

Removing the freeze in nbd_add_socket to speed up nbd device startup.

Signed-off-by: Yang Erkun <yangerkun@huawei.com>
---
 drivers/block/nbd.c | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 7ec85f94f742..1a31e621b6f7 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -1272,7 +1272,6 @@ static int nbd_add_socket(struct nbd_device *nbd, unsigned long arg,
 	struct socket *sock;
 	struct nbd_sock **socks;
 	struct nbd_sock *nsock;
-	unsigned int memflags;
 	int err;
 
 	/* Arg will be cast to int, check it to avoid overflow */
@@ -1290,12 +1289,6 @@ static int nbd_add_socket(struct nbd_device *nbd, unsigned long arg,
 		return err;
 	nbd_reclassify_socket(sock);
 
-	/*
-	 * We need to make sure we don't get any errant requests while we're
-	 * reallocating the ->socks array.
-	 */
-	memflags = blk_mq_freeze_queue(nbd->disk->queue);
-
 	if (!netlink && !nbd->task_setup &&
 	    !test_bit(NBD_RT_BOUND, &config->runtime_flags))
 		nbd->task_setup = current;
@@ -1335,12 +1328,10 @@ static int nbd_add_socket(struct nbd_device *nbd, unsigned long arg,
 	INIT_WORK(&nsock->work, nbd_pending_cmd_work);
 	socks[config->num_connections++] = nsock;
 	atomic_inc(&config->live_connections);
-	blk_mq_unfreeze_queue(nbd->disk->queue, memflags);
 
 	return 0;
 
 put_socket:
-	blk_mq_unfreeze_queue(nbd->disk->queue, memflags);
 	sockfd_put(sock);
 	return err;
 }
-- 
2.52.0