When we try to transmit an skb with md_dst attached through wireguard
we hit a null pointer dereference in wg_xmit() due to the use of
dst_mtu() which calls into dst_blackhole_mtu() which in turn tries to
dereference dst->dev.

Since wireguard doesn't use md_dsts we should use skb_valid_dst(), which
checks for DST_METADATA flag, and if it's set, then falls back to
wireguard's device mtu. That gives us the best chance of transmitting
the packet; otherwise if the blackhole netdev is used we'd get
ETH_MIN_MTU.

 [  263.693506] BUG: kernel NULL pointer dereference, address: 00000000000000e0
 [  263.693908] #PF: supervisor read access in kernel mode
 [  263.694174] #PF: error_code(0x0000) - not-present page
 [  263.694424] PGD 0 P4D 0
 [  263.694653] Oops: 0000 [#1] PREEMPT SMP NOPTI
 [  263.694876] CPU: 5 PID: 951 Comm: mausezahn Kdump: loaded Not tainted 5.18.0-rc1+ #522
 [  263.695190] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1.fc35 04/01/2014
 [  263.695529] RIP: 0010:dst_blackhole_mtu+0x17/0x20
 [  263.695770] Code: 00 00 00 0f 1f 44 00 00 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 8b 47 10 48 83 e0 fc 8b 40 04 85 c0 75 09 48 8b 07 <8b> 80 e0 00 00 00 c3 66 90 0f 1f 44 00 00 48 89 d7 be 01 00 00 00
 [  263.696339] RSP: 0018:ffffa4a4422fbb28 EFLAGS: 00010246
 [  263.696600] RAX: 0000000000000000 RBX: ffff8ac9c3553000 RCX: 0000000000000000
 [  263.696891] RDX: 0000000000000401 RSI: 00000000fffffe01 RDI: ffffc4a43fb48900
 [  263.697178] RBP: ffffa4a4422fbb90 R08: ffffffff9622635e R09: 0000000000000002
 [  263.697469] R10: ffffffff9b69a6c0 R11: ffffa4a4422fbd0c R12: ffff8ac9d18b1a00
 [  263.697766] R13: ffff8ac9d0ce1840 R14: ffff8ac9d18b1a00 R15: ffff8ac9c3553000
 [  263.698054] FS:  00007f3704c337c0(0000) GS:ffff8acaebf40000(0000) knlGS:0000000000000000
 [  263.698470] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 [  263.698826] CR2: 00000000000000e0 CR3: 0000000117a5c000 CR4: 00000000000006e0
 [  263.699214] Call Trace:
 [  263.699505]  <TASK>
 [  263.699759]  wg_xmit+0x411/0x450
 [  263.700059]  ? bpf_skb_set_tunnel_key+0x46/0x2d0
 [   263.700382]  ? dev_queue_xmit_nit+0x31/0x2b0
 [  263.700719]  dev_hard_start_xmit+0xd9/0x220
 [  263.701047]  __dev_queue_xmit+0x8b9/0xd30
 [  263.701344]  __bpf_redirect+0x1a4/0x380
 [  263.701664]  __dev_queue_xmit+0x83b/0xd30
 [  263.701961]  ? packet_parse_headers+0xb4/0xf0
 [  263.702275]  packet_sendmsg+0x9a8/0x16a0
 [  263.702596]  ? _raw_spin_unlock_irqrestore+0x23/0x40
 [  263.702933]  sock_sendmsg+0x5e/0x60
 [  263.703239]  __sys_sendto+0xf0/0x160
 [  263.703549]  __x64_sys_sendto+0x20/0x30
 [  263.703853]  do_syscall_64+0x3b/0x90
 [  263.704162]  entry_SYSCALL_64_after_hwframe+0x44/0xae
 [  263.704494] RIP: 0033:0x7f3704d50506
 [  263.704789] Code: 48 c7 c0 ff ff ff ff eb b7 66 2e 0f 1f 84 00 00 00 00 00 90 41 89 ca 64 8b 04 25 18 00 00 00 85 c0 75 11 b8 2c 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 72 c3 90 55 48 83 ec 30 44 89 4c 24 2c 4c 89
 [  263.705652] RSP: 002b:00007ffe954b0b88 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
 [  263.706141] RAX: ffffffffffffffda RBX: 0000558bb259b490 RCX: 00007f3704d50506
 [  263.706544] RDX: 000000000000004a RSI: 0000558bb259b7b2 RDI: 0000000000000003
 [  263.706952] RBP: 0000000000000000 R08: 00007ffe954b0b90 R09: 0000000000000014
 [  263.707339] R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffe954b0b90
 [  263.707735] R13: 000000000000004a R14: 0000558bb259b7b2 R15: 0000000000000001
 [  263.708132]  </TASK>
 [  263.708398] Modules linked in: bridge netconsole bonding [last unloaded: bridge]
 [  263.708942] CR2: 00000000000000e0

Fixes: e7096c13 ("net: WireGuard secure network tunnel")
Link: https://github.com/cilium/cilium/issues/19428Reported-by: Martynas Pumputis <m@lambda.lt>
Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>