[PATCH net] netlink: extack needs to be reset each time through loop

[David Ahern <dsahern@gmail.com>]

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: syzbot+315fa6766d0f7c359327@syzkaller.appspotmail.com
Signed-off-by: David Ahern <dsahern@gmail.com>
---
 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 972bfe113043..ee8252d7b3d4 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2417,7 +2417,7 @@ 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;
 
@@ -2438,6 +2438,7 @@ 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;
-- 
2.11.0