FAILED: patch "[PATCH] ALSA: control: Fix UAF in snd_ctl_elem_add() on card disconnect" failed to apply to 5.15-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x d4febca239be3c91498762dd0535a87d4af1eabc
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107060049-failed-5.15-d4febca239be@kernel.org' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From d4febca239be3c91498762dd0535a87d4af1eabc Mon Sep 17 00:00:00 2001
From: Takashi Iwai <tiwai@suse.de>
Date: Tue, 29 Sep 2026 14:53:42 +0200
Subject: [PATCH] ALSA: control: Fix UAF in snd_ctl_elem_add() on card
 disconnect

A use-after-free can be triggered via SNDRV_CTL_IOCTL_ELEM_ADD when a
USB audio card is disconnected while an ELEM_ADD ioctl is in flight.

The reproducer parks the ioctl thread inside copy_from_user() using
userfaultfd, then tears down the USB device.  Unlike every other
ALSA control _user handler (ELEM_INFO, ELEM_READ, ELEM_WRITE, TLV_*),
snd_ctl_elem_add_user() never calls snd_power_ref_and_wait(), so the
parked thread holds no power reference.  snd_card_disconnect() therefore
cannot observe it via snd_power_sync_ref() and proceeds unimpeded:

  1. card->shutdown is set to 1
  2. device_del(&card->card_dev) drops the kobject reference on the
     parent USB interface device (card->dev = &intf->dev)
  3. The USB core drops its own reference and calls device_release(),
     freeing the struct usb_interface, including the embedded struct
     device that card->dev points to

When the userfaultfd is resolved and the thread resumes,
snd_ctl_elem_add() acquires controls_rwsem without checking
card->shutdown and calls __snd_ctl_add_replace().  Because the
reproducer pre-registered the same control, the CTL_ADD_EXCLUSIVE
path calls dev_err(card->dev, ...) on the freed USB interface:

  KASAN: slab-use-after-free Read in __dev_printk

Add a card->shutdown guard immediately after acquiring controls_rwsem
in snd_ctl_elem_add().  At that point card->shutdown is guaranteed to
be stable: snd_card_disconnect() sets it before freeing the parent
device, and it never transitions back to 0.  A thread that acquired
the lock before disconnect sees shutdown=0 and holds the write lock
through the rest of the operation, preventing concurrent disconnect
from proceeding past its own controls_rwsem-less shutdown=1 store
(which happened earlier, outside the lock) from racing with dev_err().

Reported-by: Farhad Alemi <farhad.alemi@berkeley.edu>
Cc: stable@vger.kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20260929125346.1478631-3-tiwai@suse.de
---
 sound/core/control.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/sound/core/control.c b/sound/core/control.c
index 4199342d4ffe0..535ceba294acf 100644
--- a/sound/core/control.c
+++ b/sound/core/control.c
@@ -1802,6 +1802,8 @@ static int snd_ctl_elem_add(struct snd_ctl_file *file,
 	alloc_size = compute_user_elem_size(private_size, count);
 
 	guard(rwsem_write)(&card->controls_rwsem);
+	if (card->shutdown)
+		return -ENODEV;
 	if (check_user_elem_overflow(card, alloc_size))
 		return -ENOMEM;
 
-- 
2.53.0