[PATCH RESEND v3] nbd_genl_status: null check for nla_nest_start

["Michal Koutný" <mkoutny@suse.com>]

From: Navid Emamdoost <navid.emamdoost@gmail.com>

nla_nest_start may fail and return NULL. The check is inserted, and
errno is selected based on other call sites within the same source code.
Update: removed extra new line.
v3 Update: added release reply, thanks to Michal Kubecek for pointing
out.

Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Link: https://lore.kernel.org/r/20190911164013.27364-1-navid.emamdoost@gmail.com/
---

I'm resending the patch because there was apparent consensus of its
inclusion and it seems it was only overlooked. Some people may care
about this because of CVE-2019-16089.

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

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 592cfa8b765a..109dccd9a515 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -2394,6 +2394,12 @@ static int nbd_genl_status(struct sk_buff *skb, struct genl_info *info)
 	}
 
 	dev_list = nla_nest_start_noflag(reply, NBD_ATTR_DEVICE_LIST);
+	if (!dev_list) {
+		nlmsg_free(reply);
+		ret = -EMSGSIZE;
+		goto out;
+	}
+
 	if (index == -1) {
 		ret = idr_for_each(&nbd_index_idr, &status_cb, reply);
 		if (ret) {
-- 
2.40.0