[PATCH net] net/smc: serialize link group free work scheduling

[Chengfeng Ye <nicoyip.dev@gmail.com>]

smc_lgr_schedule_free_work() checks lgr->freeing without the link group
list lock held by the teardown paths. A concurrent smc_lgr_free_work()
can set freeing and cancel the delayed work between that check and
mod_delayed_work(), leaving a timer pending on a freed link group:

  CPU 0 (smc_conn_free)            CPU 1 (smc_lgr_free_work)
  observe lgr->freeing == 0
                                  set lgr->freeing under lgr_lock
                                  cancel_delayed_work()
                                  smc_lgr_free(): drop lgr reference
  mod_delayed_work()
  smc_lgr_put(): drop last reference and free lgr

When the timer expires, the timer core accesses the freed memory.
KASAN reported:

  BUG: KASAN: use-after-free in __run_timers+0x86d/0x8d0
  Write of size 8 at addr ffff8881186902a8 by task swapper/2/0
  Call Trace:
   <IRQ>
   __run_timers+0x86d/0x8d0
   timer_expire_remote+0xd3/0x120
   tmigr_handle_remote_up+0x4f4/0xab0
   __walk_groups_from+0x40/0x150
   tmigr_handle_remote+0x229/0x2c0
   run_timer_softirq+0x1f5/0x250
   handle_softirqs+0x18d/0x5b0
   </IRQ>

Hold the existing link group list lock across the freeing check and
mod_delayed_work(). Rearming then either precedes the freeing transition
and is caught by the subsequent cancellation, or sees freeing set and
is skipped. smc_conn_free() retains its link group reference until after
scheduling, keeping the group alive while acquiring the lock.

Fixes: 8e316b9e7260 ("net/smc: improve link group freeing")
Cc: stable@vger.kernel.org
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
 net/smc/smc_core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
index 9974149659c2..8feb486ada56 100644
--- a/net/smc/smc_core.c
+++ b/net/smc/smc_core.c
@@ -80,6 +80,10 @@ static void smc_ibdev_cnt_dec(struct smc_link *lnk)
 
 static void smc_lgr_schedule_free_work(struct smc_link_group *lgr)
 {
+	spinlock_t *lgr_lock; /* protects lgr->freeing */
+
+	smc_lgr_list_head(lgr, &lgr_lock);
+	spin_lock_bh(lgr_lock);
 	/* client link group creation always follows the server link group
 	 * creation. For client use a somewhat higher removal delay time,
 	 * otherwise there is a risk of out-of-sync link groups.
@@ -90,6 +94,7 @@ static void smc_lgr_schedule_free_work(struct smc_link_group *lgr)
 						SMC_LGR_FREE_DELAY_CLNT :
 						SMC_LGR_FREE_DELAY_SERV);
 	}
+	spin_unlock_bh(lgr_lock);
 }
 
 /* Register connection's alert token in our lookup structure.
-- 
2.43.0