Re: [syzbot] [usb?] KASAN: slab-use-after-free Write in usb_free_urb

[Michal Pecio]

On Tue, 01 Sep 2026 22:09:33 -0700, syzbot wrote:
> syzbot has found a reproducer for the following issue on:
> 
> HEAD commit:    786262be6048 Merge tag 'edac_updates_for_v7.3_rc2' of git:..
> git tree:       upstream
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=12a28125580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=b454dc6b1b7acd30
> dashboard link: https://syzkaller.appspot.com/bug?extid=832ce9fa3face1b7d44d
> compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1735af79580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=135b7f79580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/c3e528ebf709/disk-786262be.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/46f3418d2828/vmlinux-786262be.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/60ab8a27c4f7/bzImage-786262be.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+832ce9fa3face1b7d44d@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> usb 1-1: received EP1 urb->status = -2
> ==================================================================
> BUG: KASAN: slab-use-after-free in instrument_atomic_read_write include/linux/instrumented.h:112 [inline]
> BUG: KASAN: slab-use-after-free in atomic_fetch_sub_release include/linux/atomic/atomic-instrumented.h:400 [inline]
> BUG: KASAN: slab-use-after-free in __refcount_sub_and_test include/linux/refcount.h:389 [inline]
> BUG: KASAN: slab-use-after-free in __refcount_dec_and_test include/linux/refcount.h:432 [inline]
> BUG: KASAN: slab-use-after-free in refcount_dec_and_test include/linux/refcount.h:450 [inline]
> BUG: KASAN: slab-use-after-free in kref_put include/linux/kref.h:64 [inline]
> BUG: KASAN: slab-use-after-free in usb_free_urb+0x24/0x120 drivers/usb/core/urb.c:96
> Write of size 4 at addr ffff88803cee1050 by task ktimers/1/29
> 
> CPU: 1 UID: 0 PID: 29 Comm: ktimers/1 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
> 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
>  check_region_inline mm/kasan/generic.c:-1 [inline]
>  kasan_check_range+0x264/0x2c0 mm/kasan/generic.c:200
>  instrument_atomic_read_write include/linux/instrumented.h:112 [inline]
>  atomic_fetch_sub_release include/linux/atomic/atomic-instrumented.h:400 [inline]
>  __refcount_sub_and_test include/linux/refcount.h:389 [inline]
>  __refcount_dec_and_test include/linux/refcount.h:432 [inline]
>  refcount_dec_and_test include/linux/refcount.h:450 [inline]
>  kref_put include/linux/kref.h:64 [inline]
>  usb_free_urb+0x24/0x120 drivers/usb/core/urb.c:96
>  dummy_timer+0xaac/0x4d50 drivers/usb/gadget/udc/dummy_hcd.c:2019
>  __run_hrtimer kernel/time/hrtimer.c:2067 [inline]
>  __hrtimer_run_queues+0x3eb/0xaf0 kernel/time/hrtimer.c:2124
>  hrtimer_run_softirq+0x1e1/0x2e0 kernel/time/hrtimer.c:2141
>  handle_softirqs+0x1da/0x6d0 kernel/softirq.c:645
>  __do_softirq kernel/softirq.c:679 [inline]
>  run_ktimerd+0x6a/0x100 kernel/softirq.c:1183
>  smpboot_thread_fn+0x565/0xa70 kernel/smpboot.c:160
>  kthread+0x38b/0x470 kernel/kthread.c:436
>  ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>

[Adding sound people]

So what happens here is that USB core continues to use a URB after
completion to implement things like usb_kill_urb(), so URBs are
reference counted. Then core decrements the count - one more use.

If a driver waits for completion or even usb_kill_urb() to return
and then proceeds to free the URB's storage, this becomes a UAF.
This driver embeds 2 URBs in its priv and does just that.

A URB can only exist as an independent allocation, core will free
it if upon finding zero reference count in such case.

This is a mechanical conversion, untested. I don't have the HW.

#syz test

diff --git a/sound/usb/caiaq/device.c b/sound/usb/caiaq/device.c
index a16e59248480..3587794207d4 100644
--- a/sound/usb/caiaq/device.c
+++ b/sound/usb/caiaq/device.c
@@ -192,8 +192,7 @@ static void usb_ep1_command_reply_dispatch (struct urb* urb)
 		break;
 	}
 
-	cdev->ep1_in_urb.actual_length = 0;
-	ret = usb_submit_urb(&cdev->ep1_in_urb, GFP_ATOMIC);
+	ret = usb_submit_urb(cdev->ep1_in_urb, GFP_ATOMIC);
 	if (ret < 0)
 		dev_err(dev, "unable to submit urb. OOM!?\n");
 }
@@ -403,6 +402,8 @@ static void card_free(struct snd_card *card)
 {
 	struct snd_usb_caiaqdev *cdev = caiaqdev(card);
 
+	usb_free_urb(cdev->ep1_in_urb);
+	usb_free_urb(cdev->midi_out_urb);
 #ifdef CONFIG_SND_USB_CAIAQ_INPUT
 	snd_usb_caiaq_input_free(cdev);
 #endif
@@ -457,22 +458,24 @@ static int init_card(struct snd_usb_caiaqdev *cdev)
 		return -EIO;
 	}
 
-	usb_init_urb(&cdev->ep1_in_urb);
-	usb_init_urb(&cdev->midi_out_urb);
+	cdev->ep1_in_urb = usb_alloc_urb(0, GFP_KERNEL);
+	cdev->midi_out_urb = usb_alloc_urb(0, GFP_KERNEL);
+	if (!cdev->ep1_in_urb || !cdev->midi_out_urb)
+		return -ENOMEM;
 
-	usb_fill_bulk_urb(&cdev->ep1_in_urb, usb_dev,
+	usb_fill_bulk_urb(cdev->ep1_in_urb, usb_dev,
 			  usb_rcvbulkpipe(usb_dev, 0x1),
 			  cdev->ep1_in_buf, EP1_BUFSIZE,
 			  usb_ep1_command_reply_dispatch, cdev);
 
-	usb_fill_bulk_urb(&cdev->midi_out_urb, usb_dev,
+	usb_fill_bulk_urb(cdev->midi_out_urb, usb_dev,
 			  usb_sndbulkpipe(usb_dev, 0x1),
 			  cdev->midi_out_buf, EP1_BUFSIZE,
 			  snd_usb_caiaq_midi_output_done, cdev);
 
 	/* sanity checks of EPs before actually submitting */
-	if (usb_urb_ep_type_check(&cdev->ep1_in_urb) ||
-	    usb_urb_ep_type_check(&cdev->midi_out_urb)) {
+	if (usb_urb_ep_type_check(cdev->ep1_in_urb) ||
+	    usb_urb_ep_type_check(cdev->midi_out_urb)) {
 		dev_err(dev, "invalid EPs\n");
 		return -EINVAL;
 	}
@@ -480,7 +483,7 @@ static int init_card(struct snd_usb_caiaqdev *cdev)
 	init_waitqueue_head(&cdev->ep1_wait_queue);
 	init_waitqueue_head(&cdev->prepare_wait_queue);
 
-	if (usb_submit_urb(&cdev->ep1_in_urb, GFP_KERNEL) != 0)
+	if (usb_submit_urb(cdev->ep1_in_urb, GFP_KERNEL) != 0)
 		return -EIO;
 
 	err = snd_usb_caiaq_send_command(cdev, EP1_CMD_GET_DEVICE_INFO, NULL, 0);
@@ -530,7 +533,7 @@ static int init_card(struct snd_usb_caiaqdev *cdev)
 	return 0;
 
  err_kill_urb:
-	usb_kill_urb(&cdev->ep1_in_urb);
+	usb_kill_urb(cdev->ep1_in_urb);
 	return err;
 }
 
@@ -576,8 +579,8 @@ static void snd_disconnect(struct usb_interface *intf)
 #endif
 	snd_usb_caiaq_audio_disconnect(cdev);
 
-	usb_kill_urb(&cdev->ep1_in_urb);
-	usb_kill_urb(&cdev->midi_out_urb);
+	usb_kill_urb(cdev->ep1_in_urb);
+	usb_kill_urb(cdev->midi_out_urb);
 
 	snd_card_free_when_closed(card);
 }
diff --git a/sound/usb/caiaq/device.h b/sound/usb/caiaq/device.h
index 743eb0387b5f..1c6f34693fa8 100644
--- a/sound/usb/caiaq/device.h
+++ b/sound/usb/caiaq/device.h
@@ -60,8 +60,8 @@ struct snd_usb_caiaq_cb_info;
 struct snd_usb_caiaqdev {
 	struct snd_usb_audio chip;
 
-	struct urb ep1_in_urb;
-	struct urb midi_out_urb;
+	struct urb *ep1_in_urb;
+	struct urb *midi_out_urb;
 	struct urb **data_urbs_in;
 	struct urb **data_urbs_out;
 	struct snd_usb_caiaq_cb_info *data_cb_info;
diff --git a/sound/usb/caiaq/midi.c b/sound/usb/caiaq/midi.c
index c656d0162432..18529484c8dc 100644
--- a/sound/usb/caiaq/midi.c
+++ b/sound/usb/caiaq/midi.c
@@ -43,7 +43,7 @@ static int snd_usb_caiaq_midi_output_close(struct snd_rawmidi_substream *substre
 {
 	struct snd_usb_caiaqdev *cdev = substream->rmidi->private_data;
 	if (cdev->midi_out_active) {
-		usb_kill_urb(&cdev->midi_out_urb);
+		usb_kill_urb(cdev->midi_out_urb);
 		cdev->midi_out_active = 0;
 	}
 	return 0;
@@ -64,9 +64,9 @@ static void snd_usb_caiaq_midi_send(struct snd_usb_caiaqdev *cdev,
 		return;
 
 	cdev->midi_out_buf[2] = len;
-	cdev->midi_out_urb.transfer_buffer_length = len+3;
+	cdev->midi_out_urb->transfer_buffer_length = len+3;
 
-	ret = usb_submit_urb(&cdev->midi_out_urb, GFP_ATOMIC);
+	ret = usb_submit_urb(cdev->midi_out_urb, GFP_ATOMIC);
 	if (ret < 0)
 		dev_err(dev,
 			"snd_usb_caiaq_midi_send(%p): usb_submit_urb() failed,"