ip6_tunnel: fix possible NULL deref in ip6_tnl_xmit

Make sure to test that skb has a dst attached to it.

general protection fault, probably for non-canonical address 0xdffffc0000000011: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000088-0x000000000000008f]
CPU: 0 PID: 32650 Comm: syz-executor.4 Not tainted 5.17.0-rc2-next-20220204-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:ip6_tnl_xmit+0x2140/0x35f0 net/ipv6/ip6_tunnel.c:1127
Code: 4d 85 f6 0f 85 c5 04 00 00 e8 9c b0 66 f9 48 83 e3 fe 48 b8 00 00 00 00 00 fc ff df 48 8d bb 88 00 00 00 48 89 fa 48 c1 ea 03 <0f> b6 04 02 84 c0 74 07 7f 05 e8 11 25 b2 f9 44 0f b6 b3 88 00 00
RSP: 0018:ffffc900141b7310 EFLAGS: 00010206
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffc9000c77a000
RDX: 0000000000000011 RSI: ffffffff8811f854 RDI: 0000000000000088
RBP: ffffc900141b7480 R08: 0000000000000000 R09: 0000000000000008
R10: ffffffff8811f846 R11: 0000000000000008 R12: ffffc900141b7548
R13: ffff8880297c6000 R14: 0000000000000000 R15: ffff8880351c8dc0
FS:  00007f9827ba2700(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b31322000 CR3: 0000000033a70000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 ipxip6_tnl_xmit net/ipv6/ip6_tunnel.c:1386 [inline]
 ip6_tnl_start_xmit+0x71e/0x1830 net/ipv6/ip6_tunnel.c:1435
 __netdev_start_xmit include/linux/netdevice.h:4683 [inline]
 netdev_start_xmit include/linux/netdevice.h:4697 [inline]
 xmit_one net/core/dev.c:3473 [inline]
 dev_hard_start_xmit+0x1eb/0x920 net/core/dev.c:3489
 __dev_queue_xmit+0x2a24/0x3760 net/core/dev.c:4116
 packet_snd net/packet/af_packet.c:3057 [inline]
 packet_sendmsg+0x2265/0x5460 net/packet/af_packet.c:3084
 sock_sendmsg_nosec net/socket.c:705 [inline]
 sock_sendmsg+0xcf/0x120 net/socket.c:725
 sock_write_iter+0x289/0x3c0 net/socket.c:1061
 call_write_iter include/linux/fs.h:2075 [inline]
 do_iter_readv_writev+0x47a/0x750 fs/read_write.c:726
 do_iter_write+0x188/0x710 fs/read_write.c:852
 vfs_writev+0x1aa/0x630 fs/read_write.c:925
 do_writev+0x27f/0x300 fs/read_write.c:968
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7f9828c2d059

Fixes: c1f55c5e ("ip6_tunnel: allow routing IPv4 traffic in NBMA mode")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Qing Deng <i@moy.cat>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_tunnel.c

@@ -1122,7 +1122,10 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev, __u8 dsfield,
 			memcpy(&fl6->daddr, addr6, sizeof(fl6->daddr));
 			neigh_release(neigh);
 		} else if (skb->protocol == htons(ETH_P_IP)) {
-			struct rtable *rt = skb_rtable(skb);
+			const struct rtable *rt = skb_rtable(skb);
+
+			if (!rt)
+				goto tx_err_link_failure;
 
 			if (rt->rt_gw_family == AF_INET6)
 				memcpy(&fl6->daddr, &rt->rt_gw6, sizeof(fl6->daddr));