[PATCH net v2 1/2] net/smc: serialize link group free work scheduling

[Chengfeng Ye]

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.

  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(). Set freeing under that lock in early cleanup as well,
before __smc_lgr_terminate() cancels the work. An SMC-D server releases
smc_server_lgr_pending before receiving CONFIRM, so another connection can
reuse the group and race its removal with early cleanup.

Give freeing its own storage. The neighbouring sync_err and terminating
bitfields are written without the list lock; their read-modify-write
updates must not overwrite the freeing transition.

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@xxxxxxxxxxxxxxx
Assisted-by: GPT-6.1-Sol
Signed-off-by: Chengfeng Ye <nicoyip.dev@xxxxxxxxx>
---
Changes in v2:
- Give freeing its own storage so adjacent bitfield updates cannot
  overwrite the list-lock-protected transition.
- Set freeing under the list lock during early cleanup to prevent a
  concurrent SMC-D connection from rearming after cancellation.
- Keep asynchronous cancellation without adding work-owned references.
- Send with the early-cleanup ownership fix as an ordered two-patch series.

v1: https://lore.kernel.org/r/20260927074520.3694663-1-nicoyip.dev@xxxxxxxxx/

 net/smc/smc_core.c | 8 +++++++-
 net/smc/smc_core.h | 2 +-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
index 9974149659c2..478c8bc2759a 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.
@@ -691,6 +696,7 @@ void smc_lgr_cleanup_early(struct smc_link_group *lgr)
 	/* do not use this link group for new connections */
 	if (!list_empty(&lgr->list))
 		list_del_init(&lgr->list);
+	lgr->freeing = 1;
 	spin_unlock_bh(lgr_lock);
 	__smc_lgr_terminate(lgr, true);
 }
@@ -1565,7 +1571,7 @@ static void __smc_lgr_terminate(struct smc_link_group *lgr, bool soft)
 
 	if (lgr->terminating)
 		return;	/* lgr already terminating */
-	/* cancel free_work sync, will terminate when lgr->freeing is set */
+	/* cancel pending free_work; a running instance rechecks freeing */
 	cancel_delayed_work(&lgr->free_work);
 	lgr->terminating = 1;
 
diff --git a/net/smc/smc_core.h b/net/smc/smc_core.h
index 5c18f08a4c8a..b8c67f277337 100644
--- a/net/smc/smc_core.h
+++ b/net/smc/smc_core.h
@@ -303,7 +303,7 @@ struct smc_link_group {
 	struct workqueue_struct	*tx_wq;		/* wq for conn. tx workers */
 	u8			sync_err : 1;	/* lgr no longer fits to peer */
 	u8			terminating : 1;/* lgr is terminating */
-	u8			freeing : 1;	/* lgr is being freed */
+	bool			freeing;	/* lgr is being freed */
 
 	refcount_t		refcnt;		/* lgr reference count */
 	bool			is_smcd;	/* SMC-R or SMC-D */
-- 
2.43.0