net/smc: fix possible NULL deref in smc_pnet_add_eth()

net/smc: fix possible NULL deref in smc_pnet_add_eth()

I missed that @ndev value can be NULL.

I prefer not factorizing this NULL check, and instead
clearly document where a NULL might be expected.

general protection fault, probably for non-canonical address 0xdffffc00000000ba: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x00000000000005d0-0x00000000000005d7]
CPU: 0 PID: 19875 Comm: syz-executor.2 Not tainted 5.16.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:__lock_acquire+0xd7a/0x5470 kernel/locking/lockdep.c:4897
Code: 14 0e 41 bf 01 00 00 00 0f 86 c8 00 00 00 89 05 5c 20 14 0e e9 bd 00 00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 f2 48 c1 ea 03 <80> 3c 02 00 0f 85 9f 2e 00 00 49 81 3e 20 c5 1a 8f 0f 84 52 f3 ff
RSP: 0018:ffffc900057071d0 EFLAGS: 00010002
RAX: dffffc0000000000 RBX: 1ffff92000ae0e65 RCX: 1ffff92000ae0e4c
RDX: 00000000000000ba RSI: 0000000000000000 RDI: 0000000000000001
RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000001
R10: fffffbfff1b24ae2 R11: 000000000008808a R12: 0000000000000000
R13: ffff888040ca4000 R14: 00000000000005d0 R15: 0000000000000000
FS:  00007fbd683e0700(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b2be22000 CR3: 0000000013fea000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 lock_acquire kernel/locking/lockdep.c:5637 [inline]
 lock_acquire+0x1ab/0x510 kernel/locking/lockdep.c:5602
 __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
 _raw_spin_lock_irqsave+0x39/0x50 kernel/locking/spinlock.c:162
 ref_tracker_alloc+0x182/0x440 lib/ref_tracker.c:84
 netdev_tracker_alloc include/linux/netdevice.h:3859 [inline]
 smc_pnet_add_eth net/smc/smc_pnet.c:372 [inline]
 smc_pnet_enter net/smc/smc_pnet.c:492 [inline]
 smc_pnet_add+0x49a/0x14d0 net/smc/smc_pnet.c:555
 genl_family_rcv_msg_doit+0x228/0x320 net/netlink/genetlink.c:731
 genl_family_rcv_msg net/netlink/genetlink.c:775 [inline]
 genl_rcv_msg+0x328/0x580 net/netlink/genetlink.c:792
 netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2494
 genl_rcv+0x24/0x40 net/netlink/genetlink.c:803
 netlink_unicast_kernel net/netlink/af_netlink.c:1317 [inline]
 netlink_unicast+0x539/0x7e0 net/netlink/af_netlink.c:1343
 netlink_sendmsg+0x904/0xe00 net/netlink/af_netlink.c:1919
 sock_sendmsg_nosec net/socket.c:705 [inline]
 sock_sendmsg+0xcf/0x120 net/socket.c:725
 ____sys_sendmsg+0x6e8/0x810 net/socket.c:2413
 ___sys_sendmsg+0xf3/0x170 net/socket.c:2467
 __sys_sendmsg+0xe5/0x1b0 net/socket.c:2496
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Fixes: b60645248af3 ("net/smc: add net device tracker to struct smc_pnetentry")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>