[SRU E/F/U] UBUNTU: SAUCE: nbd_genl_status: null check for nla_nest_start

[Thadeu Lima de Souza Cascardo]

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

CVE-2019-16089

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 at gmail.com>
Reviewed-by: Michal Kubecek <mkubecek at suse.cz>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 drivers/block/nbd.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 43cff01a5a67..1b1eaae6e639 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -2277,6 +2277,12 @@ 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) {
-- 
2.25.1