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ALSA: usb-audio: fix use-after-free in ump_to_endpoint()

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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: ff49d1df79 ("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>
Link: https://patch.msgid.link/20260726051337.41124-1-baul.lee@xbow.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
This commit is contained in:
Baul Lee 2026-07-26 14:13:37 +09:00 committed by Takashi Iwai
parent cd74e5cba4
commit 4a05b2d1b4
1 changed files with 3 additions and 1 deletions
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4
sound/usb/midi2.c
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@ -329,7 +329,7 @@ ump_to_endpoint(struct snd_ump_endpoint *ump, int dir)
{
struct snd_usb_midi2_ump *rmidi = ump->private_data;
return rmidi->eps[dir];
return rmidi ? rmidi->eps[dir] : NULL;
}
/* ump open callback */
@ -685,6 +685,8 @@ static void free_all_midi2_umps(struct snd_usb_midi2_interface *umidi)
rmidi = list_first_entry(&umidi->rawmidi_list,
struct snd_usb_midi2_ump, list);
list_del(&rmidi->list);
if (rmidi->ump)
rmidi->ump->private_data = NULL;
kfree(rmidi);
}
}