ip: Return drop reason if in_dev is NULL in ip_route_input_rcu().

syzkaller reported a warning in __sk_skb_reason_drop().

Commit 61b95c7 ("net: ip: make ip_route_input_rcu() return
drop reasons") missed a path where -EINVAL is returned.

Then, the cited commit started to trigger the warning with the
invalid error.

Let's fix it by returning SKB_DROP_REASON_NOT_SPECIFIED.

[0]:
WARNING: CPU: 0 PID: 10 at net/core/skbuff.c:1216 __sk_skb_reason_drop net/core/skbuff.c:1216 [inline]
WARNING: CPU: 0 PID: 10 at net/core/skbuff.c:1216 sk_skb_reason_drop+0x97/0x1b0 net/core/skbuff.c:1241
Modules linked in:
CPU: 0 UID: 0 PID: 10 Comm: kworker/0:1 Not tainted 6.12.0-10686-gbb18265c3aba #10 1c308307628619808b5a4a0495c4aab5637b0551
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
Workqueue: wg-crypt-wg2 wg_packet_decrypt_worker
RIP: 0010:__sk_skb_reason_drop net/core/skbuff.c:1216 [inline]
RIP: 0010:sk_skb_reason_drop+0x97/0x1b0 net/core/skbuff.c:1241
Code: 5d 41 5c 41 5d 41 5e e9 e7 9e 95 fd e8 e2 9e 95 fd 31 ff 44 89 e6 e8 58 a1 95 fd 45 85 e4 0f 85 a2 00 00 00 e8 ca 9e 95 fd 90 <0f> 0b 90 e8 c1 9e 95 fd 44 89 e6 bf 01 00 00 00 e8 34 a1 95 fd 41
RSP: 0018:ffa0000000007650 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 000000000000ffff RCX: ffffffff83bc3592
RDX: ff110001002a0000 RSI: ffffffff83bc34d6 RDI: 0000000000000007
RBP: ff11000109ee85f0 R08: 0000000000000001 R09: ffe21c00213dd0da
R10: 000000000000ffff R11: 0000000000000000 R12: 00000000ffffffea
R13: 0000000000000000 R14: ff11000109ee86d4 R15: ff11000109ee8648
FS:  0000000000000000(0000) GS:ff1100011a000000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020177000 CR3: 0000000108a3d006 CR4: 0000000000771ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe07f0 DR7: 0000000000000600
PKRU: 55555554
Call Trace:
 <IRQ>
 kfree_skb_reason include/linux/skbuff.h:1263 [inline]
 ip_rcv_finish_core.constprop.0+0x896/0x2320 net/ipv4/ip_input.c:424
 ip_list_rcv_finish.constprop.0+0x1b2/0x710 net/ipv4/ip_input.c:610
 ip_sublist_rcv net/ipv4/ip_input.c:636 [inline]
 ip_list_rcv+0x34a/0x460 net/ipv4/ip_input.c:670
 __netif_receive_skb_list_ptype net/core/dev.c:5715 [inline]
 __netif_receive_skb_list_core+0x536/0x900 net/core/dev.c:5762
 __netif_receive_skb_list net/core/dev.c:5814 [inline]
 netif_receive_skb_list_internal+0x77c/0xdc0 net/core/dev.c:5905
 gro_normal_list include/net/gro.h:515 [inline]
 gro_normal_list include/net/gro.h:511 [inline]
 napi_complete_done+0x219/0x8c0 net/core/dev.c:6256
 wg_packet_rx_poll+0xbff/0x1e40 drivers/net/wireguard/receive.c:488
 __napi_poll.constprop.0+0xb3/0x530 net/core/dev.c:6877
 napi_poll net/core/dev.c:6946 [inline]
 net_rx_action+0x9eb/0xe30 net/core/dev.c:7068
 handle_softirqs+0x1ac/0x740 kernel/softirq.c:554
 do_softirq kernel/softirq.c:455 [inline]
 do_softirq+0x48/0x80 kernel/softirq.c:442
 </IRQ>
 <TASK>
 __local_bh_enable_ip+0xed/0x110 kernel/softirq.c:382
 spin_unlock_bh include/linux/spinlock.h:396 [inline]
 ptr_ring_consume_bh include/linux/ptr_ring.h:367 [inline]
 wg_packet_decrypt_worker+0x3ba/0x580 drivers/net/wireguard/receive.c:499
 process_one_work+0x940/0x1a70 kernel/workqueue.c:3229
 process_scheduled_works kernel/workqueue.c:3310 [inline]
 worker_thread+0x639/0xe30 kernel/workqueue.c:3391
 kthread+0x283/0x350 kernel/kthread.c:389
 ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:244
 </TASK>

Fixes: 82d9983 ("net: ip: make ip_route_input_noref() return drop reasons")
Reported-by: syzkaller <[email protected]>
Signed-off-by: Kuniyuki Iwashima <[email protected]>
Reviewed-by: David Ahern <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

net/ipv4/route.c

@@ -2478,7 +2478,8 @@ ip_route_input_rcu(struct sk_buff *skb, __be32 daddr, __be32 saddr,
 		int our = 0;
 
 		if (!in_dev)
-			return -EINVAL;
+			return reason;
+
 		our = ip_check_mc_rcu(in_dev, daddr, saddr,
 				      ip_hdr(skb)->protocol);