Re: [PATCH] ALSA: us144mkii: re-anchor capture URBs on resubmission

On Tue, 04 Aug 2026 14:36:25 +0200,
Baul Lee wrote:
> 
> capture_urb_complete() resubmits each capture URB without anchoring it:
> 
> 	usb_get_urb(urb);
> 	ret = usb_submit_urb(urb, GFP_ATOMIC);
> 
> Anchoring is a property of a submission, not of the URB.  The giveback
> path calls usb_unanchor_urb() before urb->complete(), so an URB
> resubmitted from its own completion handler is off the anchor.  The
> capture URBs are anchored once, at stream start, so from the first
> completion onward tascam->capture_anchor is empty.
> 
> tascam_free_urbs(), tascam_disconnect(), tascam_suspend() and the
> stop-work path all call usb_kill_anchored_urbs(&tascam->capture_anchor)
> to reap the capture URBs before anything is freed.  With the anchor empty
> those calls return immediately and the URBs stay queued on the host
> controller.
> 
> tascam_free_urbs() then returns the capture transfer buffers with
> usb_free_coherent(), and snd_card_free() releases the snd_card
> allocation that embeds tascam (card->private_data).  The controller
> completes the queued URBs afterwards, writing device-supplied data into
> the freed transfer buffer, and capture_urb_complete() dereferences the
> freed driver object.
> 
> KASAN on 7.2.0-rc5 (arm64):
> 
>   BUG: KASAN: slab-use-after-free in dummy_timer
>   Write of size 512 at addr ffff000015b62000
>    __asan_memcpy
>    dummy_timer
>    hrtimer_run_softirq
>   Allocated by task 64:
>    usb_alloc_coherent
>    tascam_alloc_urbs
>    tascam_probe
>   Freed by task 170:
>    usb_free_coherent
>    tascam_free_urbs
>    tascam_disconnect
>    usb_unbind_interface
> 
>   BUG: KASAN: slab-use-after-free in capture_urb_complete
>   Read of size 4 at addr ffff0000170ee878
>   Freed by task 170:
>    release_card_device
>    snd_card_free
>    tascam_disconnect
> 
> Restore the usb_anchor_urb() between the reference count bump and the
> resubmission.  That also makes the handler's usb_unanchor_urb() failure
> arm meaningful again and restores usb_kill_anchored_urbs() as a barrier
> on the disconnect, suspend and stop-work paths.
> 
> The anchoring was removed on the premise that the URB is already anchored
> from the initial submission, which does not hold once the first giveback
> has run.
> 
> Discovered by XBOW, triaged by Baul Lee <[email protected]>
> 
> Fixes: 5cff1529a2f9 ("ALSA: us144mkii: capture_urb_complete: redundant usb_anchor_urb corrupts anchor list on each resubmission")
> Reported-by: Federico Kirschbaum <[email protected]>
> Reported-by: Baul Lee <[email protected]>
> Cc: [email protected]
> Signed-off-by: Baul Lee <[email protected]>

Applied now.  Thanks.


Takashi