Re: [PATCH v4] ALSA: caiaq: Decoupling ep1_in_urb in caiaq dev

[Takashi Iwai]

On Thu, 03 Sep 2026 15:05:21 +0200,
Edward Adam Davis wrote:
> 
> The epq_in_urb object belonging to the caiaq device is coupled within
> the struct snd_usb_caiaqdev. After usb_submit_urb(epq_in_urb, GFP_KERNEL)
> executes successfully, epq_in_urb is successfully added to the urbp_list
> queue of the dummy HCD driver (userspace specifies dummy_hcd as the HCD
> layer driver for the caiaq USB device).
> 
> When init_card() calls snd_usb_caiaq_send_command() which subsequently
> fails due to a timeout, and proceeds to call snd_card_free() to release
> the card, the embedded ep1_in_urb object is also freed. When the dummy
> HCD driver detects that the URB has been unlinked, it returns the URB
> (by usb_hcd_giveback_urb()), which triggers [1].
> 
> Decouple the ep1_in_urb object from the struct snd_usb_caiaqdev and switch
> to using a pointer instead. Separately allocate and manage the memory for
> ep1_in_urb to prevent the release of the snd_card memory object from
> interfering with it.
> 
> midi_out_urb has the same issue as ep1_in_urb and is handled in the same
> way.
> 
> [1]
> BUG: KASAN: slab-use-after-free in usb_free_urb+0x24/0x120 drivers/usb/core/urb.c:96
> Write of size 4 at addr ffff88803cee1050 by task ktimers/1/29
> Call Trace:
>  usb_free_urb+0x24/0x120 drivers/usb/core/urb.c:96
>  dummy_timer+0xaac/0x4d50 drivers/usb/gadget/udc/dummy_hcd.c:2019
>  __run_hrtimer kernel/time/hrtimer.c:2067 [inline]
>  __hrtimer_run_queues+0x3eb/0xaf0 kernel/time/hrtimer.c:2124
>  hrtimer_run_softirq+0x1e1/0x2e0 kernel/time/hrtimer.c:2141
> 
> Allocated by task 36:
>  snd_card_new+0x7b/0x110 sound/core/init.c:184
>  create_card sound/usb/caiaq/device.c:429 [inline]
>  snd_probe+0x236/0x1af0 sound/usb/caiaq/device.c:544
> 
> Freed by task 36:
>  snd_card_free_when_closed sound/core/init.c:630 [inline]
>  snd_card_free+0x138/0x1d0 sound/core/init.c:662
>  snd_probe+0x162b/0x1af0 sound/usb/caiaq/device.c:553
>  
> Fixes: 523f1dce3743 ("[ALSA] Add Native Instrument usb audio device support")
> Reported-by: syzbot+832ce9fa3face1b7d44d@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=832ce9fa3face1b7d44d
> Tested-by: syzbot+832ce9fa3face1b7d44d@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Edward Adam Davis <eadavis@xxxxxxxx>

Applied now.  Thanks.


Takashi