ip6_dst_idev() can return NULL, xfrm6_get_saddr() must act accordingly.

syzbot reported:

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 1 PID: 12 Comm: kworker/u8:1 Not tainted 6.10.0-rc2-syzkaller-00383-gb8481381 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/02/2024
Workqueue: wg-kex-wg1 wg_packet_handshake_send_worker
 RIP: 0010:xfrm6_get_saddr+0x93/0x130 net/ipv6/xfrm6_policy.c:64
Code: df 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 97 00 00 00 4c 8b ab d8 00 00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 ea 48 c1 ea 03 <80> 3c 02 00 0f 85 86 00 00 00 4d 8b 6d 00 e8 ca 13 47 01 48 b8 00
RSP: 0018:ffffc90000117378 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff88807b079dc0 RCX: ffffffff89a0d6d7
RDX: 0000000000000000 RSI: ffffffff89a0d6e9 RDI: ffff88807b079e98
RBP: ffff88807ad73248 R08: 0000000000000007 R09: fffffffffffff000
R10: ffff88807b079dc0 R11: 0000000000000007 R12: ffffc90000117480
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8880b9300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f4586d00440 CR3: 0000000079042000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
  xfrm_get_saddr net/xfrm/xfrm_policy.c:2452 [inline]
  xfrm_tmpl_resolve_one net/xfrm/xfrm_policy.c:2481 [inline]
  xfrm_tmpl_resolve+0xa26/0xf10 net/xfrm/xfrm_policy.c:2541
  xfrm_resolve_and_create_bundle+0x140/0x2570 net/xfrm/xfrm_policy.c:2835
  xfrm_bundle_lookup net/xfrm/xfrm_policy.c:3070 [inline]
  xfrm_lookup_with_ifid+0x4d1/0x1e60 net/xfrm/xfrm_policy.c:3201
  xfrm_lookup net/xfrm/xfrm_policy.c:3298 [inline]
  xfrm_lookup_route+0x3b/0x200 net/xfrm/xfrm_policy.c:3309
  ip6_dst_lookup_flow+0x15c/0x1d0 net/ipv6/ip6_output.c:1256
  send6+0x611/0xd20 drivers/net/wireguard/socket.c:139
  wg_socket_send_skb_to_peer+0xf9/0x220 drivers/net/wireguard/socket.c:178
  wg_socket_send_buffer_to_peer+0x12b/0x190 drivers/net/wireguard/socket.c:200
  wg_packet_send_handshake_initiation+0x227/0x360 drivers/net/wireguard/send.c:40
  wg_packet_handshake_send_worker+0x1c/0x30 drivers/net/wireguard/send.c:51
  process_one_work+0x9fb/0x1b60 kernel/workqueue.c:3231
  process_scheduled_works kernel/workqueue.c:3312 [inline]
  worker_thread+0x6c8/0xf70 kernel/workqueue.c:3393
  kthread+0x2c1/0x3a0 kernel/kthread.c:389
  ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240615154231.234442-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>