Message-ID: <2025072226-stadium-spearhead-63cb@gregkh>
Date: Tue, 22 Jul 2025 14:31:15 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Jens Axboe <axboe@...nel.dk>
Cc: syzbot <syzbot+01523a0ae5600aef5895@...kaller.appspotmail.com>,
	io-uring@...r.kernel.org, linux-kernel@...r.kernel.org,
	syzkaller-bugs@...glegroups.com, abbotti@....co.uk,
	hsweeten@...ionengravers.com
Subject: Re: [syzbot] [io-uring?] KASAN: slab-use-after-free Read in
 io_poll_remove_entries

On Tue, Jul 22, 2025 at 06:21:50AM -0600, Jens Axboe wrote:
> On 7/20/25 1:00 PM, Jens Axboe wrote:
> > On 7/20/25 12:49 PM, Jens Axboe wrote:
> >> On 7/20/25 12:24 PM, Jens Axboe wrote:
> >>> On 7/19/25 11:29 AM, syzbot wrote:
> >>>> Hello,
> >>>>
> >>>> syzbot found the following issue on:
> >>>>
> >>>> HEAD commit:    4871b7cb27f4 Merge tag 'v6.16-rc6-smb3-client-fixes' of gi..
> >>>> git tree:       upstream
> >>>> console output: https://syzkaller.appspot.com/x/log.txt?x=1288c38c580000
> >>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=fa738a4418f051ee
> >>>> dashboard link: https://syzkaller.appspot.com/bug?extid=01523a0ae5600aef5895
> >>>> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> >>>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1688c38c580000
> >>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=166ed7d4580000
> >>>>
> >>>> Downloadable assets:
> >>>> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-4871b7cb.raw.xz
> >>>> vmlinux: https://storage.googleapis.com/syzbot-assets/4a9dea51d821/vmlinux-4871b7cb.xz
> >>>> kernel image: https://storage.googleapis.com/syzbot-assets/f96c723cdfe6/bzImage-4871b7cb.xz
> >>>>
> >>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> >>>> Reported-by: syzbot+01523a0ae5600aef5895@...kaller.appspotmail.com
> >>>>
> >>>> ==================================================================
> >>>> BUG: KASAN: slab-use-after-free in __raw_spin_lock_irq include/linux/spinlock_api_smp.h:119 [inline]
> >>>> BUG: KASAN: slab-use-after-free in _raw_spin_lock_irq+0x36/0x50 kernel/locking/spinlock.c:170
> >>>> Read of size 1 at addr ffff88803c6f42b0 by task kworker/2:2/1339
> >>>>
> >>>> CPU: 2 UID: 0 PID: 1339 Comm: kworker/2:2 Not tainted 6.16.0-rc6-syzkaller-00253-g4871b7cb27f4 #0 PREEMPT(full) 
> >>>> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> >>>> Workqueue: events io_fallback_req_func
> >>>> Call Trace:
> >>>>  <TASK>
> >>>>  __dump_stack lib/dump_stack.c:94 [inline]
> >>>>  dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
> >>>>  print_address_description mm/kasan/report.c:378 [inline]
> >>>>  print_report+0xcd/0x610 mm/kasan/report.c:480
> >>>>  kasan_report+0xe0/0x110 mm/kasan/report.c:593
> >>>>  __kasan_check_byte+0x36/0x50 mm/kasan/common.c:557
> >>>>  kasan_check_byte include/linux/kasan.h:399 [inline]
> >>>>  lock_acquire kernel/locking/lockdep.c:5845 [inline]
> >>>>  lock_acquire+0xfc/0x350 kernel/locking/lockdep.c:5828
> >>>>  __raw_spin_lock_irq include/linux/spinlock_api_smp.h:119 [inline]
> >>>>  _raw_spin_lock_irq+0x36/0x50 kernel/locking/spinlock.c:170
> >>>>  spin_lock_irq include/linux/spinlock.h:376 [inline]
> >>>>  io_poll_remove_entry io_uring/poll.c:146 [inline]
> >>>>  io_poll_remove_entries.part.0+0x14e/0x7e0 io_uring/poll.c:179
> >>>>  io_poll_remove_entries io_uring/poll.c:159 [inline]
> >>>>  io_poll_task_func+0x4cd/0x1130 io_uring/poll.c:326
> >>>>  io_fallback_req_func+0x1c7/0x6d0 io_uring/io_uring.c:259
> >>>>  process_one_work+0x9cf/0x1b70 kernel/workqueue.c:3238
> >>>>  process_scheduled_works kernel/workqueue.c:3321 [inline]
> >>>>  worker_thread+0x6c8/0xf10 kernel/workqueue.c:3402
> >>>>  kthread+0x3c5/0x780 kernel/kthread.c:464
> >>>>  ret_from_fork+0x5d4/0x6f0 arch/x86/kernel/process.c:148
> >>>>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> >>>>  </TASK>
> >>>>
> >>>> Allocated by task 6154:
> >>>>  kasan_save_stack+0x33/0x60 mm/kasan/common.c:47
> >>>>  kasan_save_track+0x14/0x30 mm/kasan/common.c:68
> >>>>  poison_kmalloc_redzone mm/kasan/common.c:377 [inline]
> >>>>  __kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:394
> >>>>  kmalloc_noprof include/linux/slab.h:905 [inline]
> >>>>  kzalloc_noprof include/linux/slab.h:1039 [inline]
> >>>>  __comedi_device_postconfig_async drivers/comedi/drivers.c:664 [inline]
> >>>>  __comedi_device_postconfig drivers/comedi/drivers.c:721 [inline]
> >>>>  comedi_device_postconfig+0x2cb/0xc80 drivers/comedi/drivers.c:756
> >>>>  comedi_device_attach+0x3cf/0x900 drivers/comedi/drivers.c:998
> >>>>  do_devconfig_ioctl+0x1a7/0x580 drivers/comedi/comedi_fops.c:855
> >>>>  comedi_unlocked_ioctl+0x15bb/0x2e90 drivers/comedi/comedi_fops.c:2136
> >>>>  vfs_ioctl fs/ioctl.c:51 [inline]
> >>>>  __do_sys_ioctl fs/ioctl.c:907 [inline]
> >>>>  __se_sys_ioctl fs/ioctl.c:893 [inline]
> >>>>  __x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:893
> >>>>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> >>>>  do_syscall_64+0xcd/0x4c0 arch/x86/entry/syscall_64.c:94
> >>>>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >>>>
> >>>> Freed by task 6156:
> >>>>  kasan_save_stack+0x33/0x60 mm/kasan/common.c:47
> >>>>  kasan_save_track+0x14/0x30 mm/kasan/common.c:68
> >>>>  kasan_save_free_info+0x3b/0x60 mm/kasan/generic.c:576
> >>>>  poison_slab_object mm/kasan/common.c:247 [inline]
> >>>>  __kasan_slab_free+0x51/0x70 mm/kasan/common.c:264
> >>>>  kasan_slab_free include/linux/kasan.h:233 [inline]
> >>>>  slab_free_hook mm/slub.c:2381 [inline]
> >>>>  slab_free mm/slub.c:4643 [inline]
> >>>>  kfree+0x2b4/0x4d0 mm/slub.c:4842
> >>>>  comedi_device_detach_cleanup drivers/comedi/drivers.c:171 [inline]
> >>>>  comedi_device_detach+0x2a4/0x9e0 drivers/comedi/drivers.c:208
> >>>>  do_devconfig_ioctl+0x46c/0x580 drivers/comedi/comedi_fops.c:833
> >>>>  comedi_unlocked_ioctl+0x15bb/0x2e90 drivers/comedi/comedi_fops.c:2136
> >>>>  vfs_ioctl fs/ioctl.c:51 [inline]
> >>>>  __do_sys_ioctl fs/ioctl.c:907 [inline]
> >>>>  __se_sys_ioctl fs/ioctl.c:893 [inline]
> >>>>  __x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:893
> >>>>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> >>>>  do_syscall_64+0xcd/0x4c0 arch/x86/entry/syscall_64.c:94
> >>>>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >>>
> >>> I took a quick look at this, and surely looks like a comedi bug. If you
> >>> call the ioctl part (do_devconfig_ioctl()) with a NULL arg, it just does
> >>> a detach and frees the device, regardless of whether anyone has it
> >>> opened or not?! It's got some odd notion of checking whether it's busy
> >>> or not. For this case, someone has a poll active on the device, yet it
> >>> still happily frees it.
> >>>
> >>> CC'ing some folks, as this looks utterly broken.
> >>
> >> Case in point, I added:
> >>
> >> diff --git a/drivers/comedi/drivers.c b/drivers/comedi/drivers.c
> >> index 376130bfba8a..4d5fde012558 100644
> >> --- a/drivers/comedi/drivers.c
> >> +++ b/drivers/comedi/drivers.c
> >> @@ -167,6 +167,7 @@ static void comedi_device_detach_cleanup(struct comedi_device *dev)
> >>  				kfree(s->private);
> >>  			comedi_free_subdevice_minor(s);
> >>  			if (s->async) {
> >> +				WARN_ON_ONCE(waitqueue_active(&s->async->wait_head));
> >>  				comedi_buf_alloc(dev, s, 0);
> >>  				kfree(s->async);
> >>  			}
> >>
> >> and this is the first thing that triggers:
> >>
> >> WARNING: CPU: 1 PID: 807 at drivers/comedi/drivers.c:170 comedi_device_detach+0x510/0x720
> >> Modules linked in:
> >> CPU: 1 UID: 0 PID: 807 Comm: comedi Not tainted 6.16.0-rc6-00281-gf4a40a4282f4-dirty #1438 NONE 
> >> Hardware name: linux,dummy-virt (DT)
> >> pstate: 21400005 (nzCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
> >> pc : comedi_device_detach+0x510/0x720
> >> lr : comedi_device_detach+0x1dc/0x720
> >> sp : ffff80008aeb7880
> >> x29: ffff80008aeb7880 x28: 1fffe00020251205 x27: ffff000101289028
> >> x26: ffff00010578a000 x25: ffff000101289000 x24: 0000000000000007
> >> x23: 1fffe00020af1437 x22: 1fffe00020af1438 x21: 0000000000000000
> >> x20: 0000000000000000 x19: dfff800000000000 x18: ffff0000db102ec0
> >> x17: ffff80008208e6dc x16: ffff80008362e120 x15: ffff800080a47c1c
> >> x14: ffff8000826f5aec x13: ffff8000836a0cc4 x12: ffff700010adcd15
> >> x11: 1ffff00010adcd14 x10: ffff700010adcd14 x9 : ffff8000836a105c
> >> x8 : ffff800085bc0cc0 x7 : ffff00000b035b50 x6 : 0000000000000000
> >> x5 : 0000000000000000 x4 : ffff800080960e08 x3 : 0000000000000001
> >> x2 : ffff00000b4bf930 x1 : 0000000000000000 x0 : ffff0000d7e2b0d8
> >> Call trace:
> >>  comedi_device_detach+0x510/0x720 (P)
> >>  do_devconfig_ioctl+0x37c/0x4b8
> >>  comedi_unlocked_ioctl+0x33c/0x2bd8
> >>  __arm64_sys_ioctl+0x124/0x1a0
> >>  invoke_syscall.constprop.0+0x60/0x2a0
> >>  el0_svc_common.constprop.0+0x148/0x240
> >>  do_el0_svc+0x40/0x60
> >>  el0_svc+0x44/0xe0
> >>  el0t_64_sync_handler+0x104/0x130
> >>  el0t_64_sync+0x170/0x178
> >>
> >> Not sure what the right fix for comedi is here, it'd probably be at
> >> least somewhat saner if it only allowed removal of the device when the
> >> ref count would be 1 (for the ioctl itself). Just ignoring the file ref
> >> and allowing blanket removal seems highly suspicious / broken.
> >>
> >> As there's no comedi subsystem in syzbot, moving it to kernel:
> >>
> >> #syz set subsystems: kernel
> > 
> > Something like the below may help, at least it'll tell us the device is
> > busy if there's a poll active on it.
> > 
> > #syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > 
> > 
> > diff --git a/drivers/comedi/comedi_fops.c b/drivers/comedi/comedi_fops.c
> > index 3383a7ce27ff..ea96bc4b818e 100644
> > --- a/drivers/comedi/comedi_fops.c
> > +++ b/drivers/comedi/comedi_fops.c
> > @@ -785,21 +785,31 @@ void comedi_device_cancel_all(struct comedi_device *dev)
> >  static int is_device_busy(struct comedi_device *dev)
> >  {
> >  	struct comedi_subdevice *s;
> > -	int i;
> > +	int i, is_busy = 0;
> >  
> >  	lockdep_assert_held(&dev->mutex);
> >  	if (!dev->attached)
> >  		return 0;
> >  
> > +	/* prevent new polls */
> > +	down_write(&dev->attach_lock);
> > +
> >  	for (i = 0; i < dev->n_subdevices; i++) {
> >  		s = &dev->subdevices[i];
> > -		if (s->busy)
> > -			return 1;
> > -		if (s->async && comedi_buf_is_mmapped(s))
> > -			return 1;
> > +		if (s->busy) {
> > +			is_busy = 1;
> > +			break;
> > +		}
> > +		if (!s->async)
> > +			continue;
> > +		if (comedi_buf_is_mmapped(s) ||
> > +		    waitqueue_active(&s->async->wait_head)) {
> > +			is_busy = 1;
> > +			break;
> > +		}
> >  	}
> > -
> > -	return 0;
> > +	up_write(&dev->attach_lock);
> > +	return is_busy;
> >  }
> >  
> >  /*
> 
> Haven't heard anything back, so I guess I'll send it out as a patch?

Please do.  Note, a bunch of comedi bugfixes for syzbot-found-issues
just langed in 6.16-rc7.  I don't know if there's a way to trigger
syzbot to re-run its check on that branch before you submit this?

thanks,

greg k-h

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