[PATCH v2 net 2/3] ipv6: fix possible UAF in ip6_finish_output2()

[Eric Dumazet <edumazet@google.com>]

If skb_expand_head() returns NULL, skb has been freed
and associated dst/idev could also have been freed.

We need to hold rcu_read_lock() to make sure the dst and
associated idev are alive.

Fixes: 5796015fa968 ("ipv6: allocate enough headroom in ip6_finish_output2()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Vasily Averin <vasily.averin@linux.dev>
---
 net/ipv6/ip6_output.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index f7b53effc80f8b3b7a839f58097d021a11f9eb37..1b9ebee7308f02a626c766de1794e6b114ae8554 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -70,11 +70,15 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
 
 	/* Be paranoid, rather than too clever. */
 	if (unlikely(hh_len > skb_headroom(skb)) && dev->header_ops) {
+		/* Make sure idev stays alive */
+		rcu_read_lock();
 		skb = skb_expand_head(skb, hh_len);
 		if (!skb) {
 			IP6_INC_STATS(net, idev, IPSTATS_MIB_OUTDISCARDS);
+			rcu_read_unlock();
 			return -ENOMEM;
 		}
+		rcu_read_unlock();
 	}
 
 	hdr = ipv6_hdr(skb);
-- 
2.46.0.184.g6999bdac58-goog