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* [PATCH v2] ALSA: 6fire: fix OOB write from device-reported iso length
@ 2026-09-14  7:43 Xiang Mei
  2026-09-14  9:46 ` Takashi Iwai
  0 siblings, 1 reply; 2+ messages in thread
From: Xiang Mei @ 2026-09-14  7:43 UTC (permalink / raw)
  To: perex, tiwai, torsten.schenk
  Cc: co+855929c2df672879, linux-sound, linux-kernel, 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 limit the frame count to what fits in rt->out_packet_size, the OUT
endpoint's wMaxPacketSize.  This bounds total_length by the buffer size
while keeping each packet length aligned to a whole output frame.

  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: LLM
Signed-off-by: Xiang Mei <xmei5@asu.edu>
---
 sound/usb/6fire/pcm.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/sound/usb/6fire/pcm.c b/sound/usb/6fire/pcm.c
index 21789db6657d..0285d79ace0f 100644
--- a/sound/usb/6fire/pcm.c
+++ b/sound/usb/6fire/pcm.c
@@ -335,11 +335,19 @@ 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 frames = 0;
+
 		isoc_out = &out_urb->instance->iso_frame_desc[i];
 		isoc_in = &in_urb->instance->iso_frame_desc[i];
+		if (isoc_in->actual_length > 4)
+			frames = (isoc_in->actual_length - 4)
+					/ (rt->in_n_analog << 2);
+		frames = min_t(unsigned int, frames,
+				       (rt->out_packet_size - 4)
+				       / (rt->out_n_analog << 2));
+
 		isoc_out->offset = total_length;
-		isoc_out->length = (isoc_in->actual_length - 4) / (rt->in_n_analog << 2)
-				* (rt->out_n_analog << 2) + 4;
+		isoc_out->length = frames * (rt->out_n_analog << 2) + 4;
 		isoc_out->status = 0;
 		total_length += isoc_out->length;
 	}
-- 
2.43.0


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2026-09-14  7:43 [PATCH v2] ALSA: 6fire: fix OOB write from device-reported iso length Xiang Mei
2026-09-14  9:46 ` Takashi Iwai

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