[PATCH net] ipv6: fix possible UAF in icmpv6_rcv()

[Eric Dumazet <edumazet@google.com>]

Caching saddr and daddr before pskb_pull() is problematic
since skb->head can change.

Remove these temporary variables:

- We only access &ipv6_hdr(skb)->saddr and &ipv6_hdr(skb)->daddr
  when net_dbg_ratelimited() is called in the slow path.

- Avoid potential future misuse after pskb_pull() call.

Fixes: 4b3418fba0fe ("ipv6: icmp: include addresses in debug messages")
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/ipv6/icmp.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/net/ipv6/icmp.c b/net/ipv6/icmp.c
index 799d9e9ac45d11f7b460da7d8a7aeeaf0eb50f2f..efb23807a0262e8d68aa1afc8d96ee94eab89d50 100644
--- a/net/ipv6/icmp.c
+++ b/net/ipv6/icmp.c
@@ -1104,7 +1104,6 @@ static int icmpv6_rcv(struct sk_buff *skb)
 	struct net *net = dev_net_rcu(skb->dev);
 	struct net_device *dev = icmp6_dev(skb);
 	struct inet6_dev *idev = __in6_dev_get(dev);
-	const struct in6_addr *saddr, *daddr;
 	struct icmp6hdr *hdr;
 	u8 type;
 
@@ -1135,12 +1134,10 @@ static int icmpv6_rcv(struct sk_buff *skb)
 
 	__ICMP6_INC_STATS(dev_net_rcu(dev), idev, ICMP6_MIB_INMSGS);
 
-	saddr = &ipv6_hdr(skb)->saddr;
-	daddr = &ipv6_hdr(skb)->daddr;
-
 	if (skb_checksum_validate(skb, IPPROTO_ICMPV6, ip6_compute_pseudo)) {
 		net_dbg_ratelimited("ICMPv6 checksum failed [%pI6c > %pI6c]\n",
-				    saddr, daddr);
+				    &ipv6_hdr(skb)->saddr,
+				    &ipv6_hdr(skb)->daddr);
 		goto csum_error;
 	}
 
@@ -1220,7 +1217,8 @@ static int icmpv6_rcv(struct sk_buff *skb)
 			break;
 
 		net_dbg_ratelimited("icmpv6: msg of unknown type [%pI6c > %pI6c]\n",
-				    saddr, daddr);
+				    &ipv6_hdr(skb)->saddr,
+				    &ipv6_hdr(skb)->daddr);
 
 		/*
 		 * error of unknown type.
-- 
2.54.0.rc1.513.gad8abe7a5a-goog