[PATCH net] ipv4: fix use-after-free in ip_cmsg_recv_dstaddr()

[Eric Dumazet]

Caching ip_hdr(skb) before a call to pskb_may_pull() is buggy,
do not do it.

Fixes: 2efd4fca703a ("ip: in cmsg IP(V6)_ORIGDSTADDR call pskb_may_pull")
Signed-off-by: Eric Dumazet <eduma...@google.com>
Cc: Willem de Bruijn <will...@google.com>
Reported-by: syzbot <syzkal...@googlegroups.com>
---
 net/ipv4/ip_sockglue.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index 
c0fe5ad996f238091f5b9585adb586a571f653f0..26c36cccabdc2c8cc95cfd609672d412c493fc42
 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -149,7 +149,6 @@ static void ip_cmsg_recv_security(struct msghdr *msg, 
struct sk_buff *skb)
 static void ip_cmsg_recv_dstaddr(struct msghdr *msg, struct sk_buff *skb)
 {
        struct sockaddr_in sin;
-       const struct iphdr *iph = ip_hdr(skb);
        __be16 *ports;
        int end;
 
@@ -164,7 +163,7 @@ static void ip_cmsg_recv_dstaddr(struct msghdr *msg, struct 
sk_buff *skb)
        ports = (__be16 *)skb_transport_header(skb);
 
        sin.sin_family = AF_INET;
-       sin.sin_addr.s_addr = iph->daddr;
+       sin.sin_addr.s_addr = ip_hdr(skb)->daddr;
        sin.sin_port = ports[1];
        memset(sin.sin_zero, 0, sizeof(sin.sin_zero));
 
-- 
2.19.0.605.g01d371f741-goog