net: tcp: check skb is non-NULL for exact match on lookups

Andrey reported the following error report while running the syzkaller
fuzzer:

general protection fault: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 648 Comm: syz-executor Not tainted 4.9.0-rc3+ #333
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
task: ffff8800398c4480 task.stack: ffff88003b468000
RIP: 0010:[<ffffffff83091106>]  [<     inline     >]
inet_exact_dif_match include/net/tcp.h:808
RIP: 0010:[<ffffffff83091106>]  [<ffffffff83091106>]
__inet_lookup_listener+0xb6/0x500 net/ipv4/inet_hashtables.c:219
RSP: 0018:ffff88003b46f270  EFLAGS: 00010202
RAX: 0000000000000004 RBX: 0000000000004242 RCX: 0000000000000001
RDX: 0000000000000000 RSI: ffffc90000e3c000 RDI: 0000000000000054
RBP: ffff88003b46f2d8 R08: 0000000000004000 R09: ffffffff830910e7
R10: 0000000000000000 R11: 000000000000000a R12: ffffffff867fa0c0
R13: 0000000000004242 R14: 0000000000000003 R15: dffffc0000000000
FS:  00007fb135881700(0000) GS:ffff88003ec00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020cc3000 CR3: 000000006d56a000 CR4: 00000000000006f0
Stack:
 0000000000000000 000000000601a8c0 0000000000000000 ffffffff00004242
 424200003b9083c2 ffff88003def4041 ffffffff84e7e040 0000000000000246
 ffff88003a0911c0 0000000000000000 ffff88003a091298 ffff88003b9083ae
Call Trace:
 [<ffffffff831100f4>] tcp_v4_send_reset+0x584/0x1700 net/ipv4/tcp_ipv4.c:643
 [<ffffffff83115b1b>] tcp_v4_rcv+0x198b/0x2e50 net/ipv4/tcp_ipv4.c:1718
 [<ffffffff83069d22>] ip_local_deliver_finish+0x332/0xad0
net/ipv4/ip_input.c:216
...

MD5 has a code path that calls __inet_lookup_listener with a null skb,
so inet{6}_exact_dif_match needs to check skb against null before pulling
the flag.

Fixes: a04a480d ("net: Require exact match for TCP socket lookups if
       dif is l3mdev")
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ipv6.h

@@ -149,7 +149,7 @@ static inline bool inet6_exact_dif_match(struct net *net, struct sk_buff *skb)
 {
 #if defined(CONFIG_NET_L3_MASTER_DEV)
 	if (!net->ipv4.sysctl_tcp_l3mdev_accept &&
-	    ipv6_l3mdev_skb(IP6CB(skb)->flags))
+	    skb && ipv6_l3mdev_skb(IP6CB(skb)->flags))
 		return true;
 #endif
 	return false;

include/net/tcp.h

@@ -805,7 +805,7 @@ static inline bool inet_exact_dif_match(struct net *net, struct sk_buff *skb)
 {
 #if IS_ENABLED(CONFIG_NET_L3_MASTER_DEV)
 	if (!net->ipv4.sysctl_tcp_l3mdev_accept &&
-	    ipv4_l3mdev_skb(TCP_SKB_CB(skb)->header.h4.flags))
+	    skb && ipv4_l3mdev_skb(TCP_SKB_CB(skb)->header.h4.flags))
 		return true;
 #endif
 	return false;