Subject: [PATCH 28/37] netlink: extack needs to be reset each time through loop

From: David Ahern <[email-masked]>
Date: Wed, 10 Jan 2018 13:00:39 -0800

[ Upstream commit cbbdf8433a5f117b1a2119ea30fc651b61ef7570 ]

syzbot triggered the WARN_ON in netlink_ack testing the bad_attr value.
The problem is that netlink_rcv_skb loops over the skb repeatedly invoking
the callback and without resetting the extack leaving potentially stale
data. Initializing each time through avoids the WARN_ON.

Fixes: 2d4bc93368f5a ("netlink: extended ACK reporting")
Reported-by: [email-masked]
Signed-off-by: David Ahern <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/netlink/af_netlink.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index aac9d68b4636..f4eb3fe59fe5 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2393,7 +2393,7 @@ int netlink_rcv_skb(struct sk_buff *skb, int (*cb)(struct sk_buff *,
 						   struct nlmsghdr *,
 						   struct netlink_ext_ack *))
 {
-	struct netlink_ext_ack extack = {};
+	struct netlink_ext_ack extack;
 	struct nlmsghdr *nlh;
 	int err;
 
@@ -2414,6 +2414,7 @@ int netlink_rcv_skb(struct sk_buff *skb, int (*cb)(struct sk_buff *,
 		if (nlh->nlmsg_type < NLMSG_MIN_TYPE)
 			goto ack;
 
+		memset(&extack, 0, sizeof(extack));
 		err = cb(skb, nlh, &extack);
 		if (err == -EINTR)
 			goto skip;
-- 
2.14.3