[PATCH net 1/2] wireguard: device: fix metadata_dst xmit null pointer dereference

[Nikolay Aleksandrov <razor@blackwall.org>]

When we try to transmit an skb with md_dst attached through wireguard
we hit a null pointer dereference[1] 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
fallback 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.

[1] calltrace:
 [  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

CC: stable@vger.kernel.org
CC: wireguard@lists.zx2c4.com
CC: Jason A. Donenfeld <Jason@zx2c4.com>
CC: Daniel Borkmann <daniel@iogearbox.net>
CC: Martynas Pumputis <m@lambda.lt>
Fixes: e7096c131e51 ("net: WireGuard secure network tunnel")
Reported-by: Martynas Pumputis <m@lambda.lt>
Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
---
 drivers/net/wireguard/device.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wireguard/device.c b/drivers/net/wireguard/device.c
index 0fad1331303c..aa9a7a5970fd 100644
--- a/drivers/net/wireguard/device.c
+++ b/drivers/net/wireguard/device.c
@@ -19,6 +19,7 @@
 #include <linux/if_arp.h>
 #include <linux/icmp.h>
 #include <linux/suspend.h>
+#include <net/dst_metadata.h>
 #include <net/icmp.h>
 #include <net/rtnetlink.h>
 #include <net/ip_tunnels.h>
@@ -167,7 +168,7 @@ static netdev_tx_t wg_xmit(struct sk_buff *skb, struct net_device *dev)
 		goto err_peer;
 	}
 
-	mtu = skb_dst(skb) ? dst_mtu(skb_dst(skb)) : dev->mtu;
+	mtu = skb_valid_dst(skb) ? dst_mtu(skb_dst(skb)) : dev->mtu;
 
 	__skb_queue_head_init(&packets);
 	if (!skb_is_gso(skb)) {
-- 
2.35.1