Re: [PATCH] ALSA: line6: Clear prev_fbuf on capture buffer release

[Takashi Iwai]

On Tue, 13 Oct 2026 08:35:54 +0200,
Yuanzhe Liu wrote:
> 
> From: YuanZhe Liu <25031212351@xxxxxxxxxxxxxxxxx>
> 
> A KASAN slab-use-after-free read was reported in submit_audio_out_urb()
> when generating an impulse test signal:
> 
>   BUG: KASAN: slab-use-after-free in create_impulse_test_signal sound/usb/line6/playback.c:76 [inline]
>   BUG: KASAN: slab-use-after-free in submit_audio_out_urb+0x1848/0x1b20 sound/usb/line6/playback.c:249
>   Read of size 2 at addr ffff888008d74870 by task syz.2.3214/7785
> 
>   Allocated by task 11:
>    kmem_cache_alloc_noprof+0x2f0/0x390 mm/slub.c:4144
>    line6_buffer_acquire+0x1a1/0x220 sound/usb/line6/pcm.c:158
>    line6_pcm_acquire+0xb6/0x230 sound/usb/line6/pcm.c:329
>    line6_startup_work+0x3c/0x50 sound/usb/line6/driver.c:728
> 
>   Freed by task 7785:
>    kfree+0xf8/0x390 mm/slub.c:6566
>    line6_buffer_release sound/usb/line6/pcm.c:176 [inline]
>    line6_buffer_release+0xa5/0xf0 sound/usb/line6/pcm.c:170
>    __line6_pcm_release sound/usb/line6/pcm.c:305 [inline]
>    line6_pcm_release+0x5a/0x80 sound/usb/line6/pcm.c:313
>    snd_toneport_monitor_put+0x11b/0x190 sound/usb/line6/toneport.c:183
> 
> line6pcm->prev_fbuf points into the capture PCM stream buffer
> (line6pcm->in.buffer), which is updated during audio_in_callback() to
> feed rate synchronization, software monitoring, and impulse test signal
> generation on playback side.
> 
> When all capture streams are closed, line6_buffer_release() frees
> pstr->buffer with kfree().  However, it does not clear line6pcm->prev_fbuf.
> If playback is subsequently started (e.g. via impulse test signal
> volume control snd_line6_impulse_volume_put()), submit_audio_out_urb()
> dereferences the stale pointer line6pcm->prev_fbuf under in.lock,
> resulting in a slab-use-after-free read.
> 
> Clear prev_fbuf and prev_fsize in line6_buffer_release() when releasing
> the capture stream buffer under in.lock, ensuring that no stale buffer
> pointer can be accessed by submit_audio_out_urb().
> 
> Fixes: 63e20df1e5b2 ("ALSA: line6: Reorganize PCM stream handling")
> Cc: stable@xxxxxxxxxxxxxxx
> Co-developed-by: Bin Yu <byu@xxxxxxxxxxxxx>
> Signed-off-by: Bin Yu <byu@xxxxxxxxxxxxx>
> Co-developed-by: Cheng Wen <wencheng@xxxxxxxxxxxxx>
> Signed-off-by: Cheng Wen <wencheng@xxxxxxxxxxxxx>
> Signed-off-by: YuanZhe Liu <25031212351@xxxxxxxxxxxxxxxxx>

Applied to for-next branch.  Thanks.


Takashi