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

[Aswin Karuvally <aswin@linux.ibm.com>]

On 30/09/26 12:32, Nagamani PV wrote:
> 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);

Reviewed-by: Aswin Karuvally <aswin@linux.ibm.com>