[PATCH] HID: multitouch: stop the release timer from being rearmed on remove

[Aldo Ariel Panzardo <qwe.aldo@gmail.com>]

mt_remove() quiesces the sticky-finger timer before stopping the
hardware:

    timer_delete_sync(&td->release_timer);

    sysfs_remove_group(&hdev->dev.kobj, &mt_attribute_group);
    hid_hw_stop(hdev);

timer_delete_sync() waits for a running callback and dequeues the timer,
but it does not stop the timer from being armed again. The transport is
still delivering reports at that point, and the report path rearms it:

    if (app->quirks & MT_QUIRK_STICKY_FINGERS) {
        if (td->mt_io_flags & MT_IO_SLOTS_MASK)
            mod_timer(&td->release_timer,
                      jiffies + msecs_to_jiffies(100));

A report that arrives after timer_delete_sync() has returned therefore
leaves the timer queued. td is allocated with devm_kzalloc() against
hdev->dev, so it is freed when the driver is unbound, after mt_remove()
returns. When the timer fires afterwards, mt_expired_timeout()
dereferences the freed td:

    struct mt_device *td = timer_container_of(td, t, release_timer);
    struct hid_device *hdev = td->hdev;

    if (test_and_set_bit_lock(MT_IO_FLAGS_RUNNING, &td->mt_io_flags))

Simply moving the teardown after hid_hw_stop() does not fix this on its
own, because mt_expired_timeout() calls mt_release_contacts(), which
walks hdev->inputs; the timer still has to be quiesced before
hid_hw_stop() tears the input devices down.

Use timer_shutdown_sync() instead, which additionally makes any later
mod_timer() a no-op, so neither ordering constraint has to be traded off
against the other. This is the final-teardown pattern the function was
introduced for, and hid-wiimote already uses it for the same reason.

Fixes: 4f4001bc76fd ("HID: multitouch: fix rare Win 8 cases when the touch up event gets missing")
Cc: stable@vger.kernel.org
Reported-by: Sashiko AI review <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260723224211.613112-1-you@example.com?part=1
Signed-off-by: Aldo Ariel Panzardo <qwe.aldo@gmail.com>
---
Found by code inspection after Sashiko AI review flagged the teardown
ordering while reviewing an unrelated patch of mine. I have not
reproduced the use-after-free at runtime: it needs a report to land in
the window between timer_delete_sync() returning and the device being
unbound, which I have no way to drive reliably on the hardware I have.
The window and the rearm path are visible in the code, and the fix does
not depend on the race being hit.

 drivers/hid/hid-multitouch.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/hid/hid-multitouch.c b/drivers/hid/hid-multitouch.c
index 0495152091e3..f25065b9ec66 100644
--- a/drivers/hid/hid-multitouch.c
+++ b/drivers/hid/hid-multitouch.c
@@ -2233,7 +2233,7 @@ static void mt_remove(struct hid_device *hdev)
 {
 	struct mt_device *td = hid_get_drvdata(hdev);
 
-	timer_delete_sync(&td->release_timer);
+	timer_shutdown_sync(&td->release_timer);
 
 	sysfs_remove_group(&hdev->dev.kobj, &mt_attribute_group);
 	hid_hw_stop(hdev);
-- 
2.43.0