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

[Takashi Iwai]

On Thu, 03 Sep 2026 11:24:17 +0200,
Michal Pecio wrote:
> 
> 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.

This should work, and we've received another similar patch submission
already.

So, IIUC, now the URB *must* be always allocated via usb_alloc_urb()
and an embedded URB isn't allowed?  If so, we'd need to address other
drivers, too.


thanks,

Takashi