[PATCH v2] mmc: vub300: fix use-after-free on probe failure

[Guangshuo Li <lgs201920130244@gmail.com>]

The vub300 driver lifetime-manages its controller state using
vub300->kref, with vub300_delete() freeing the mmc host when the last
reference is dropped. The probe error path after the inactivity timer has
been armed still bypasses that lifetime rule, however, and falls through
to mmc_free_host() directly if mmc_add_host() fails.

The race window is between arming the inactivity timer and reaching the
probe error unwind after mmc_add_host() fails:

        probe thread                     timer/workqueue
        ------------                     ---------------
        kref_init(&vub300->kref)         ref = 1
        kref_get(&vub300->kref)          ref = 2, timer ref
        add_timer(inactivity_timer)
        |
        |   race window
        |<---------------------------------------------------->
        |
        mmc_add_host(mmc)
                                         inactivity timer fires
                                         vub300_queue_dead_work()
                                           kref_get()          ref = 3
                                           queue_work(deadwork)
        mmc_add_host() fails
        timer_delete_sync()
        mmc_free_host(mmc)
          frees vub300
                                         deadwork runs
                                           use-after-free

timer_delete_sync() only waits for the timer callback itself. It does
not flush deadwork that the callback may already have queued. As a
result, queued deadwork can still hold a kref while the probe error path
directly frees the backing mmc host, including the vub300 storage.

Fix this by making the post-timer probe error path obey the kref lifetime
rules. After deleting the inactivity timer, drop both the timer reference
and the initial probe reference, and return without falling through to
err_free_host. If deadwork was queued, its reference keeps the object
alive until the work item drops it; otherwise the final kref_put() runs
vub300_delete() and releases the mmc host and USB resources.

Fixes: 0613ad2401f8 ("mmc: vub300: fix return value check of mmc_add_host()")
Signed-off-by: Guangshuo Li <lgs201920130244@gmail.com>
---
v2:
  - Rebase on current mainline.
  - Correct the Fixes tag.
  - Add blank lines around the early return.
  - Reword the code comment.

 drivers/mmc/host/vub300.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/mmc/host/vub300.c b/drivers/mmc/host/vub300.c
index 6c3cb2f1c9d3..1b46bb87f39a 100644
--- a/drivers/mmc/host/vub300.c
+++ b/drivers/mmc/host/vub300.c
@@ -2342,6 +2342,16 @@ static int vub300_probe(struct usb_interface *interface,
 
 err_delete_timer:
 	timer_delete_sync(&vub300->inactivity_timer);
+	/*
+	 * vub300 may have async references via queued deadwork. Drop the
+	 * timer and initial references and let the last kref put release the
+	 * host through the proper destructor.
+	 */
+	kref_put(&vub300->kref, vub300_delete);	/* timer's kref */
+	kref_put(&vub300->kref, vub300_delete);	/* init's kref */
+
+	return retval;
+
 err_free_host:
 	mmc_free_host(mmc);
 	/*
-- 
2.43.0