ipv6: prevent possible NULL deref in fib6_nh_init()

syzbot reminds us that in6_dev_get() can return NULL.

fib6_nh_init()
    ip6_validate_gw(  &idev  )
        ip6_route_check_nh(  idev  )
            *idev = in6_dev_get(dev); // can be NULL

Oops: general protection fault, probably for non-canonical address 0xdffffc00000000bc: 0000 [#1] PREEMPT SMP KASAN PTI
KASAN: null-ptr-deref in range [0x00000000000005e0-0x00000000000005e7]
CPU: 0 PID: 11237 Comm: syz-executor.3 Not tainted 6.10.0-rc2-syzkaller-00249-gbe27b896 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/07/2024
 RIP: 0010:fib6_nh_init+0x640/0x2160 net/ipv6/route.c:3606
Code: 00 00 fc ff df 4c 8b 64 24 58 48 8b 44 24 28 4c 8b 74 24 30 48 89 c1 48 89 44 24 28 48 8d 98 e0 05 00 00 48 89 d8 48 c1 e8 03 <42> 0f b6 04 38 84 c0 0f 85 b3 17 00 00 8b 1b 31 ff 89 de e8 b8 8b
RSP: 0018:ffffc900032775a0 EFLAGS: 00010202
RAX: 00000000000000bc RBX: 00000000000005e0 RCX: 0000000000000000
RDX: 0000000000000010 RSI: ffffc90003277a54 RDI: ffff88802b3a08d8
RBP: ffffc900032778b0 R08: 00000000000002fc R09: 0000000000000000
R10: 00000000000002fc R11: 0000000000000000 R12: ffff88802b3a08b8
R13: 1ffff9200064eec8 R14: ffffc90003277a00 R15: dffffc0000000000
FS:  00007f940feb06c0(0000) GS:ffff8880b9400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 00000000245e8000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
  ip6_route_info_create+0x99e/0x12b0 net/ipv6/route.c:3809
  ip6_route_add+0x28/0x160 net/ipv6/route.c:3853
  ipv6_route_ioctl+0x588/0x870 net/ipv6/route.c:4483
  inet6_ioctl+0x21a/0x280 net/ipv6/af_inet6.c:579
  sock_do_ioctl+0x158/0x460 net/socket.c:1222
  sock_ioctl+0x629/0x8e0 net/socket.c:1341
  vfs_ioctl fs/ioctl.c:51 [inline]
  __do_sys_ioctl fs/ioctl.c:907 [inline]
  __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:893
  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f940f07cea9

Fixes: 428604fb ("ipv6: do not set routes if disable_ipv6 has been enabled")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240614082002.26407-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/route.c

@@ -3603,7 +3603,7 @@ int fib6_nh_init(struct net *net, struct fib6_nh *fib6_nh,
 	if (!dev)
 		goto out;
 
-	if (idev->cnf.disable_ipv6) {
+	if (!idev || idev->cnf.disable_ipv6) {
 		NL_SET_ERR_MSG(extack, "IPv6 is disabled on nexthop device");
 		err = -EACCES;
 		goto out;