[PATCH] ALSA: 6fire: fix OOB write from device-reported iso length

[PATCH] ALSA: 6fire: fix OOB write from device-reported iso length

[Xiang Mei]

usb6fire_pcm_in_urb_handler() sizes each outgoing isochronous packet as
(actual_length - 4) / (in_n_analog << 2) * (out_n_analog << 2) + 4, where
actual_length is the unsigned length the device reported for the matching
IN packet.  A packet completed with status 0 and actual_length < 4 wraps
the subtraction to 0x7fffffec; a zero-length isochronous packet is legal
on the bus, and the preceding loop rejects only non-zero status.  The sum
reaches memset() on out_urb->buffer, a 4832-byte object from
kcalloc(PCM_MAX_PACKET_SIZE, PCM_N_PACKETS_PER_URB).

Even without the wrap the result is out of bounds: at 88.2/96 kHz the
4-in/6-out scaling turns a full 420-byte IN packet into 628, so eight
packets span 5024 bytes of that buffer.  usb_submit_urb() rejects an
over-long descriptor only after the memset() and the
usb6fire_pcm_playback() copy of user PCM data have run.

Guard the subtraction as the sibling usb6fire_pcm_capture() already does,
and clamp to rt->out_packet_size, the OUT endpoint's wMaxPacketSize,
which bounds total_length by the buffer size.

  BUG: KASAN: out-of-bounds in usb6fire_pcm_in_urb_handler (sound/usb/6fire/pcm.c:338)
  Write of size 18446744073709551456 at addr ffff88802a3d0000 by task vhci_rx/5018
  Call Trace:
   dump_stack_lvl (lib/dump_stack.c:94 lib/dump_stack.c:120)
   print_report (mm/kasan/report.c:378 mm/kasan/report.c:482)
   kasan_report (mm/kasan/report.c:595)
   kasan_check_range (mm/kasan/generic.c:186 mm/kasan/generic.c:200)
   __asan_memset (mm/kasan/shadow.c:84)
   usb6fire_pcm_in_urb_handler (sound/usb/6fire/pcm.c:338)
   __usb_hcd_giveback_urb (drivers/usb/core/hcd.c:1657)
   usb_hcd_giveback_urb (drivers/usb/core/hcd.c:1741)
   vhci_rx_loop (drivers/usb/usbip/vhci_rx.c:107 drivers/usb/usbip/vhci_rx.c:242)
   kthread (kernel/kthread.c:436)
   ret_from_fork (arch/x86/kernel/process.c:158)
   ret_from_fork_asm (arch/x86/entry/entry_64.S:245)

  Allocated by task 10:
   __kmalloc_cache_noprof (mm/slub.c:5563)
   usb6fire_pcm_init (sound/usb/6fire/pcm.c:560 sound/usb/6fire/pcm.c:595)
   usb6fire_chip_probe (sound/usb/6fire/chip.c:133)
   usb_probe_interface (drivers/usb/core/driver.c:399)

  The buggy address belongs to the object at ffff88802a3d0000
   which belongs to the cache kmalloc-8k of size 8192
  The buggy address is located 0 bytes inside of
   4832-byte region [ffff88802a3d0000, ffff88802a3d12e0)
  Kernel panic - not syncing: Fatal exception in interrupt

Fixes: c6d43ba816d1 ("ALSA: usb/6fire - Driver for TerraTec DMX 6Fire USB")
Reported-by: co+855929c2df672879@bugs.sh
Closes: https://lore.kernel.org/all/gisnub8aWGLbyZLcDCSc7zWsHonMWGcyRgt5%40bugs.sh/
Assisted-by: Claude:claude-opus-5
Signed-off-by: Xiang Mei <xmei5@asu.edu>
---
 sound/usb/6fire/pcm.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/sound/usb/6fire/pcm.c b/sound/usb/6fire/pcm.c
index d2e274b731fe..b3ecf9fa4951 100644
--- a/sound/usb/6fire/pcm.c
+++ b/sound/usb/6fire/pcm.c
@@ -328,10 +328,16 @@ static void usb6fire_pcm_in_urb_handler(struct urb *usb_urb)
 
 	/* setup out urb structure */
 	for (i = 0; i < PCM_N_PACKETS_PER_URB; i++) {
+		unsigned int length = 4;
+
+		if (in_urb->packets[i].actual_length > 4)
+			length = (in_urb->packets[i].actual_length - 4)
+					/ (rt->in_n_analog << 2)
+					* (rt->out_n_analog << 2) + 4;
+
 		out_urb->packets[i].offset = total_length;
-		out_urb->packets[i].length = (in_urb->packets[i].actual_length
-				- 4) / (rt->in_n_analog << 2)
-				* (rt->out_n_analog << 2) + 4;
+		out_urb->packets[i].length = min_t(unsigned int, length,
+						   rt->out_packet_size);
 		out_urb->packets[i].status = 0;
 		total_length += out_urb->packets[i].length;
 	}
-- 
2.43.0

Re: [PATCH] ALSA: 6fire: fix OOB write from device-reported iso length

[Takashi Iwai]

On Sun, 13 Sep 2026 02:05:15 +0200,
Xiang Mei wrote:
> 
> usb6fire_pcm_in_urb_handler() sizes each outgoing isochronous packet as
> (actual_length - 4) / (in_n_analog << 2) * (out_n_analog << 2) + 4, where
> actual_length is the unsigned length the device reported for the matching
> IN packet.  A packet completed with status 0 and actual_length < 4 wraps
> the subtraction to 0x7fffffec; a zero-length isochronous packet is legal
> on the bus, and the preceding loop rejects only non-zero status.  The sum
> reaches memset() on out_urb->buffer, a 4832-byte object from
> kcalloc(PCM_MAX_PACKET_SIZE, PCM_N_PACKETS_PER_URB).
> 
> Even without the wrap the result is out of bounds: at 88.2/96 kHz the
> 4-in/6-out scaling turns a full 420-byte IN packet into 628, so eight
> packets span 5024 bytes of that buffer.  usb_submit_urb() rejects an
> over-long descriptor only after the memset() and the
> usb6fire_pcm_playback() copy of user PCM data have run.
> 
> Guard the subtraction as the sibling usb6fire_pcm_capture() already does,
> and clamp to rt->out_packet_size, the OUT endpoint's wMaxPacketSize,
> which bounds total_length by the buffer size.
> 
>   BUG: KASAN: out-of-bounds in usb6fire_pcm_in_urb_handler (sound/usb/6fire/pcm.c:338)
>   Write of size 18446744073709551456 at addr ffff88802a3d0000 by task vhci_rx/5018
>   Call Trace:
>    dump_stack_lvl (lib/dump_stack.c:94 lib/dump_stack.c:120)
>    print_report (mm/kasan/report.c:378 mm/kasan/report.c:482)
>    kasan_report (mm/kasan/report.c:595)
>    kasan_check_range (mm/kasan/generic.c:186 mm/kasan/generic.c:200)
>    __asan_memset (mm/kasan/shadow.c:84)
>    usb6fire_pcm_in_urb_handler (sound/usb/6fire/pcm.c:338)
>    __usb_hcd_giveback_urb (drivers/usb/core/hcd.c:1657)
>    usb_hcd_giveback_urb (drivers/usb/core/hcd.c:1741)
>    vhci_rx_loop (drivers/usb/usbip/vhci_rx.c:107 drivers/usb/usbip/vhci_rx.c:242)
>    kthread (kernel/kthread.c:436)
>    ret_from_fork (arch/x86/kernel/process.c:158)
>    ret_from_fork_asm (arch/x86/entry/entry_64.S:245)
> 
>   Allocated by task 10:
>    __kmalloc_cache_noprof (mm/slub.c:5563)
>    usb6fire_pcm_init (sound/usb/6fire/pcm.c:560 sound/usb/6fire/pcm.c:595)
>    usb6fire_chip_probe (sound/usb/6fire/chip.c:133)
>    usb_probe_interface (drivers/usb/core/driver.c:399)
> 
>   The buggy address belongs to the object at ffff88802a3d0000
>    which belongs to the cache kmalloc-8k of size 8192
>   The buggy address is located 0 bytes inside of
>    4832-byte region [ffff88802a3d0000, ffff88802a3d12e0)
>   Kernel panic - not syncing: Fatal exception in interrupt
> 
> Fixes: c6d43ba816d1 ("ALSA: usb/6fire - Driver for TerraTec DMX 6Fire USB")
> Reported-by: co+855929c2df672879@bugs.sh
> Closes: https://lore.kernel.org/all/gisnub8aWGLbyZLcDCSc7zWsHonMWGcyRgt5%40bugs.sh/
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Xiang Mei <xmei5@asu.edu>

There has been already another fix for the packet size calculation
there in the upstream (already in Linus tree, too).
Could you check with it and rebase/resubmit if the fix is still
needed?


thanks,

Takashi