smc: Fix use-after-free in __pnet_find_base_ndev().

In the Linux kernel, the following vulnerability has been resolved:

smc: Fix use-after-free in __pnet_find_base_ndev().

syzbot reported use-after-free of net_device in __pnet_find_base_ndev(),
which was called during connect(). [0]

smc_pnet_find_ism_resource() fetches sk_dst_get(sk)->dev and passes
down to pnet_find_base_ndev(), where RTNL is held. Then, UAF happened
at __pnet_find_base_ndev() when the dev is first used.

This means dev had already been freed before acquiring RTNL in
pnet_find_base_ndev().

While dev is going away, dst->dev could be swapped with blackhole_netdev,
and the dev's refcnt by dst will be released.

We must hold dev's refcnt before calling smc_pnet_find_ism_resource().

Also, smc_pnet_find_roce_resource() has the same problem.

Let's use __sk_dst_get() and dst_dev_rcu() in the two functions.

[0]:
BUG: KASAN: use-after-free in __pnet_find_base_ndev+0x1b1/0x1c0 net/smc/smc_pnet.c:926
Read of size 1 at addr ffff888036bac33a by task syz.0.3632/18609

CPU: 1 UID: 0 PID: 18609 Comm: syz.0.3632 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
Call Trace:
<TASK>
dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0xca/0x240 mm/kasan/report.c:482
kasan_report+0x118/0x150 mm/kasan/report.c:595
__pnet_find_base_ndev+0x1b1/0x1c0 net/smc/smc_pnet.c:926
pnet_find_base_ndev net/smc/smc_pnet.c:946 [inline]
smc_pnet_find_ism_by_pnetid net/smc/smc_pnet.c:1103 [inline]
smc_pnet_find_ism_resource+0xef/0x390 net/smc/smc_pnet.c:1154
smc_find_ism_device net/smc/af_smc.c:1030 [inline]
smc_find_proposal_devices net/smc/af_smc.c:1115 [inline]
__smc_connect+0x372/0x1890 net/smc/af_smc.c:1545
smc_connect+0x877/0xd90 net/smc/af_smc.c:1715
__sys_connect_file net/socket.c:2086 [inline]
__sys_connect+0x313/0x440 net/socket.c:2105
__do_sys_connect net/socket.c:2111 [inline]
__se_sys_connect net/socket.c:2108 [inline]
__x64_sys_connect+0x7a/0x90 net/socket.c:2108
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f47cbf8eba9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f47ccdb1038 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 00007f47cc1d5fa0 RCX: 00007f47cbf8eba9
RDX: 0000000000000010 RSI: 0000200000000280 RDI: 000000000000000b
RBP: 00007f47cc011e19 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f47cc1d6038 R14: 00007f47cc1d5fa0 R15: 00007ffc512f8aa8
</TASK>

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff888036bacd00 pfn:0x36bac
flags: 0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
raw: 00fff00000000000 ffffea0001243d08 ffff8880b863fdc0 0000000000000000
raw: ffff888036bacd00 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as freed
page last allocated via order 2, migratetype Unmovable, gfp_mask 0x446dc0(GFP_KERNEL_ACCOUNT|__GFP_ZERO|__GFP_NOWARN|__GFP_RETRY_MAYFAIL|__GFP_COMP), pid 16741, tgid 16741 (syz-executor), ts 343313197788, free_ts 380670750466
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0x240/0x2a0 mm/page_alloc.c:1851
prep_new_page mm/page_alloc.c:1859 [inline]
get_page_from_freelist+0x21e4/0x22c0 mm/page_alloc.c:3858
__alloc_frozen_pages_noprof+0x181/0x370 mm/page_alloc.c:5148
alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2416
___kmalloc_large_node+0x5f/0x1b0 mm/slub.c:4317
__kmalloc_large_node_noprof+0x18/0x90 mm/slub.c:4348
__do_kmalloc_node mm/slub.c:4364 [inline]
__kvmalloc_node
---truncated---