[PATCH 5.4.y 5.10.y 4/4] ipv6: fix possible UAF in ip6_finish_output2()

[Harshvardhan Jha <harshvardhan.j.jha@oracle.com>]

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit e891b36de161fcd96f12ff83667473e5067b9037 ]

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>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20240820160859.3786976-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit e891b36de161fcd96f12ff83667473e5067b9037)
Signed-off-by: Harshvardhan Jha <harshvardhan.j.jha@oracle.com>
---
 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 a8475848d0382..48f926157ef8c 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -69,11 +69,15 @@ 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