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

[Yuanzhe Liu]

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>
---
 sound/usb/line6/pcm.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/sound/usb/line6/pcm.c b/sound/usb/line6/pcm.c
index 2932eaf157f4..b4c594add1ae 100644
--- a/sound/usb/line6/pcm.c
+++ b/sound/usb/line6/pcm.c
@@ -173,6 +173,14 @@ static void line6_buffer_release(struct snd_line6_pcm *line6pcm,
 	clear_bit(type, &pstr->opened);
 	if (!pstr->opened) {
 		line6_wait_clear_audio_urbs(line6pcm, pstr);
+		if (pstr == &line6pcm->in) {
+			/* prev_fbuf points into the capture buffer we are about
+			 * to free; drop it while no capture URB is in flight.
+			 */
+			guard(spinlock_irqsave)(&line6pcm->in.lock);
+			line6pcm->prev_fbuf = NULL;
+			line6pcm->prev_fsize = 0;
+		}
 		kfree(pstr->buffer);
 		pstr->buffer = NULL;
 	}
-- 
2.34.1