[PATCH] ALSA: core: Fix use-after-free in snd_card_do_free()

[syzbot]

From: Aleksandr Nogikh <nog...@google.com>

A use-after-free was detected in snd_card_do_free() when a sound card
managed by devres is unbound while a user-space application still holds an
open file descriptor.

For managed cards, the memory is allocated using devres_alloc(), and its
release function is set to __snd_card_release(), which calls
snd_card_free(). When the device is unbound, the unbind thread calls
snd_card_free(), which drops a reference to the card's device. If the user
thread still has an open file descriptor, the reference count does not
reach zero, and the unbind thread blocks on wait_for_completion(&released).

When the user thread closes the file descriptor, it drops the final
reference, invoking the device release callback release_card_device(),
which calls snd_card_do_free(). snd_card_do_free() performs cleanup and
calls complete(card->release_completion). This wakes up the unbind thread,
which returns from snd_card_free() and __snd_card_release(). The devres
core then immediately frees the memory block containing the snd_card
structure.

Meanwhile, the user thread continues execution in snd_card_do_free() and
evaluates `if (!card->managed)`. It reads the `managed` boolean from the
snd_card structure that was just freed by the unbind thread, triggering a
KASAN use-after-free.

Fix this by caching the value of card->managed in a local variable before
calling complete(). This ensures that the card pointer is not dereferenced
after the unbind thread has been woken up and potentially freed the card.

BUG: KASAN: use-after-free in snd_card_do_free sound/core/init.c:604
[inline]
BUG: KASAN: use-after-free in release_card_device+0x1ab/0x1b0
sound/core/init.c:153
Read of size 1 at addr ffff8881912ec909 by task syz-executor130/5857

Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_address_description+0x55/0x1e0 mm/kasan/report.c:378
print_report+0x58/0x70 mm/kasan/report.c:482
kasan_report+0x117/0x150 mm/kasan/report.c:595
snd_card_do_free sound/core/init.c:604 [inline]
release_card_device+0x1ab/0x1b0 sound/core/init.c:153
device_release+0xc4/0x1f0 drivers/base/core.c:-1
kobject_cleanup lib/kobject.c:689 [inline]
kobject_release lib/kobject.c:720 [inline]
kref_put include/linux/kref.h:65 [inline]
kobject_put+0x222/0x550 lib/kobject.c:737
snd_card_file_remove+0x331/0x390 sound/core/init.c:1125
snd_pcm_release+0x12c/0x160 sound/core/pcm_native.c:2986
__fput+0x418/0xa50 fs/file_table.c:512
fput_close_sync+0x11f/0x240 fs/file_table.c:617
__do_sys_close fs/open.c:1511 [inline]
__se_sys_close fs/open.c:1496 [inline]
__x64_sys_close+0x7e/0x110 fs/open.c:1496
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
</TASK>

Fixes: e8ad415b7a55 ("ALSA: core: Add managed card creation")
Assisted-by: Gemini:gemini-3.6-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: syzbot+7061d7...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7061d72c26b7daebe2b4
Link: https://syzkaller.appspot.com/ai_job?id=24752a23-f0b6-49c1-bf20-4fa89c2e7eb2
Signed-off-by: Aleksandr Nogikh <nog...@google.com>

---
diff --git a/sound/core/init.c b/sound/core/init.c
index 56dde5bd7..577338526 100644
--- a/sound/core/init.c
+++ b/sound/core/init.c
@@ -584,6 +584,8 @@ EXPORT_SYMBOL_GPL(snd_card_disconnect_sync);

static int snd_card_do_free(struct snd_card *card)
{
+ bool managed = card->managed;
+
card->releasing = true;
#if IS_ENABLED(CONFIG_SND_MIXER_OSS)
if (snd_mixer_oss_notify_callback)
@@ -601,7 +603,7 @@ static int snd_card_do_free(struct snd_card *card)
}
if (card->release_completion)
complete(card->release_completion);
- if (!card->managed)
+ if (!managed)
kfree(card);
return 0;
}


base-commit: db2ddb87143519e20a95aa36c60b36107b736a58
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[Takashi Iwai]

Applied to for-next branch. Thanks.


Takashi