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

[Simon Horman <horms@kernel.org>]

On Wed, Sep 30, 2026 at 09:02:34AM +0200, 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.

Reviewed-by: Simon Horman <horms@kernel.org>