[PATCH net v4 2/2] s390/ctcm: Fix use-after-free in channel_remove()

[Nagamani PV <nagamani@linux.ibm.com>]

channel_remove() uses fsm_deltimer() which calls timer_delete(),
returning immediately even if the timer callback is running on
another CPU. This leaves a window where fsm_expire_timer() accesses
ch->fsm after kfree_fsm().

For MPC channels the teardown has a circular dependency:

  ch->timer callback -> ctcm_chx_txretry() -> mpc_action_go_inop()
    -> tasklet_hi_schedule(&ch->ch_disc_tasklet)

  ch_tasklet -> ctcmpc_send_sweep_resp() -> fsm_addtimer(&ch->sweep_timer)

  sweep_timer callback -> fsm_addtimer(&ch->timer)

Use timer_shutdown_sync() for both timers: it waits for any running
callback and permanently prevents rearming. Shut down sweep_timer
first (its callback rearms ch->timer at ctcm_fsms.c), then
ch->timer. Only then call tasklet_kill() -- catching any tasklet
scheduled by an in-flight callback before shutdown.
kfree(ch->discontact_th) follows tasklet_kill(ch_disc_tasklet) since
ch->ccw[15].cda points to discontact_th and
mpc_action_send_discontact() starts I/O through ch->ccw[15].

timer_shutdown_sync() is also used for non-MPC ch->timer:
ctcm_chx_txretry() (CTC_STATE_TX + CTC_EVENT_TIMER) calls
fsm_addtimer(&ch->timer) on both paths.

Depends on "s390/ctcm: Fix timer corruption in fsm_addtimer()" which
replaces add_timer() with mod_timer(). mod_timer() on a shut-down
timer is discarded; without it fsm_addtimer() calls timer_setup()
defeating timer_shutdown_sync(). Backporting without that
prerequisite is unsafe.

Fixes: 293d984f0e36 ("ctcm: infrastructure for replaced ctc driver")
Cc: stable@vger.kernel.org # depends on: s390/ctcm: Fix timer corruption in fsm_addtimer()
Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20260922101913.239103-1-nagamani@linux.ibm.com?part=2
Signed-off-by: Nagamani PV <nagamani@linux.ibm.com>
---
Changes in v4:
- Shut down timers before tasklet_kill() rather than after,
  shutting down sweep_timer before ch->timer.
- Use timer_shutdown_sync() for ch->timer on both MPC and non-MPC paths.
- Name prerequisite by commit title and add structured stable tag
  dependency (Jakub Kicinski).
- Code changed; Reviewed-by and Tested-by dropped.

Changes in v3:
- Kill ch_tasklet and ch_disc_tasklet before stopping the timers and
  freeing ch->fsm: closes freed ch->fsm access from tasklet context and
  eliminates tasklet-driven sweep_timer re-arm.
- Shut down sweep_timer with timer_shutdown_sync() before deleting
  ch->timer to prevent sweep_timer callback from re-arming ch->timer.
- Move kfree(ch->discontact_th) after tasklet_kill(ch_disc_tasklet).
- Drop Reviewed-by and Tested-by; please re-review and re-test the
  new teardown order.

Changes in v2:
- Add Fixes: and Cc: stable@vger.kernel.org tags.
---
 drivers/s390/net/ctcm_main.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/drivers/s390/net/ctcm_main.c b/drivers/s390/net/ctcm_main.c
index 8b0d76a47d9f..27a22dbc9280 100644
--- a/drivers/s390/net/ctcm_main.c
+++ b/drivers/s390/net/ctcm_main.c
@@ -211,9 +211,14 @@ static void channel_remove(struct channel *ch)
 	while (*c) {
 		if (*c == ch) {
 			*c = ch->next;
-			fsm_deltimer(&ch->timer);
 			if (IS_MPC(ch))
-				fsm_deltimer(&ch->sweep_timer);
+				timer_shutdown_sync(&ch->sweep_timer.tl);
+			timer_shutdown_sync(&ch->timer.tl);
+			if (IS_MPC(ch)) {
+				tasklet_kill(&ch->ch_tasklet);
+				tasklet_kill(&ch->ch_disc_tasklet);
+				kfree(ch->discontact_th);
+			}
 
 			kfree_fsm(ch->fsm);
 			clear_normalized_cda(&ch->ccw[4]);
@@ -221,11 +226,6 @@ static void channel_remove(struct channel *ch)
 				clear_normalized_cda(&ch->ccw[1]);
 				dev_kfree_skb_any(ch->trans_skb);
 			}
-			if (IS_MPC(ch)) {
-				tasklet_kill(&ch->ch_tasklet);
-				tasklet_kill(&ch->ch_disc_tasklet);
-				kfree(ch->discontact_th);
-			}
 			kfree(ch->ccw);
 			kfree(ch->irb);
 			kfree(ch);
-- 
2.53.0