Re: [PATCH] ALSA: usb-audio: fix use-after-free in ump_to_endpoint()

[Takashi Iwai <tiwai@suse.de>]

On Sun, 26 Jul 2026 07:13:37 +0200,
Baul Lee wrote:
> 
> create_midi2_ump() registers a card-owned snd_ump_endpoint and stores a
> back-pointer to its per-interface snd_usb_midi2_ump object in
> ump->private_data, but it never installs an ump->private_free hook and
> never clears that pointer.
> 
> If a later step of snd_usb_midi_v2_create() fails, its error path calls
> free_all_midi2_umps(), which kfree()s the snd_usb_midi2_ump object while
> the already-registered endpoint keeps pointing at it.  The created
> /dev/snd/umpC*D* node stays exposed, so the first operation of any UMP
> open, ump_to_endpoint(), dereferences the dangling ump->private_data and
> reads rmidi->eps[dir] out of freed memory.
> 
> A malicious USB MIDI 2.0 device that makes creation fail after the
> endpoint is registered can thus trigger a slab use-after-free read on a
> subsequent open of the UMP node.
> 
> Clear the endpoint's back-pointer before freeing the object, and let
> ump_to_endpoint() tolerate a NULL private_data so the open/close/trigger
> callbacks fail cleanly (their callers already handle a NULL endpoint)
> instead of dereferencing a stale pointer.
> 
> Discovered by XBOW, triaged by Baul Lee <baul.lee@xbow.com>
> 
> Fixes: ff49d1df79ae ("ALSA: usb-audio: USB MIDI 2.0 UMP support")
> Reported-by: Federico Kirschbaum <federico.kirschbaum@xbow.com>
> Reported-by: Baul Lee <baul.lee@xbow.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Baul Lee <baul.lee@xbow.com>

Applied now.  Thanks.


Takashi