net/smc: fix invalid link access in dumping SMC-R connections

A crash was found when dumping SMC-R connections. It can be reproduced
by following steps:

- environment: two RNICs on both sides.
- run SMC-R between two sides, now a SMC_LGR_SYMMETRIC type link group
  will be created.
- set the first RNIC down on either side and link group will turn to
  SMC_LGR_ASYMMETRIC_LOCAL then.
- run 'smcss -R' and the crash will be triggered.

 BUG: kernel NULL pointer dereference, address: 0000000000000010
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 8000000101fdd067 P4D 8000000101fdd067 PUD 10ce46067 PMD 0
 Oops: 0000 [#1] PREEMPT SMP PTI
 CPU: 3 PID: 1810 Comm: smcss Kdump: loaded Tainted: G W   E      6.7.0-rc6+ #51
 RIP: 0010:__smc_diag_dump.constprop.0+0x36e/0x620 [smc_diag]
 Call Trace:
  <TASK>
  ? __die+0x24/0x70
  ? page_fault_oops+0x66/0x150
  ? exc_page_fault+0x69/0x140
  ? asm_exc_page_fault+0x26/0x30
  ? __smc_diag_dump.constprop.0+0x36e/0x620 [smc_diag]
  smc_diag_dump_proto+0xd0/0xf0 [smc_diag]
  smc_diag_dump+0x26/0x60 [smc_diag]
  netlink_dump+0x19f/0x320
  __netlink_dump_start+0x1dc/0x300
  smc_diag_handler_dump+0x6a/0x80 [smc_diag]
  ? __pfx_smc_diag_dump+0x10/0x10 [smc_diag]
  sock_diag_rcv_msg+0x121/0x140
  ? __pfx_sock_diag_rcv_msg+0x10/0x10
  netlink_rcv_skb+0x5a/0x110
  sock_diag_rcv+0x28/0x40
  netlink_unicast+0x22a/0x330
  netlink_sendmsg+0x240/0x4a0
  __sock_sendmsg+0xb0/0xc0
  ____sys_sendmsg+0x24e/0x300
  ? copy_msghdr_from_user+0x62/0x80
  ___sys_sendmsg+0x7c/0xd0
  ? __do_fault+0x34/0x1a0
  ? do_read_fault+0x5f/0x100
  ? do_fault+0xb0/0x110
  __sys_sendmsg+0x4d/0x80
  do_syscall_64+0x45/0xf0
  entry_SYSCALL_64_after_hwframe+0x6e/0x76

When the first RNIC is set down, the lgr->lnk[0] will be cleared and an
asymmetric link will be allocated in lgr->link[SMC_LINKS_PER_LGR_MAX - 1]
by smc_llc_alloc_alt_link(). Then when we try to dump SMC-R connections
in __smc_diag_dump(), the invalid lgr->lnk[0] will be accessed, resulting
in this issue. So fix it by accessing the right link.

Fixes: f16a7dd5 ("smc: netlink interface for SMC sockets")
Reported-by: henaumars <henaumars@sina.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=7616


Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
Reviewed-by: Tony Lu <tonylu@linux.alibaba.com>
Link: https://lore.kernel.org/r/1703662835-53416-1-git-send-email-guwen@linux.alibaba.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>