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

[Takashi Iwai <tiwai@suse.de>]

On Fri, 14 Aug 2026 14:05:43 +0200,
syzbot wrote:
> 
> From: Aleksandr Nogikh <nogikh@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+7061d72c26b7daebe2b4@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 <nogikh@google.com>

Applied to for-next branch.  Thanks.


Takashi