Message-ID: <e029eef8-2c2b-4dfe-a01f-9903f309e39e@kernel.dk>
Date: Tue, 22 Jul 2025 07:00:38 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Ian Abbott <abbotti@....co.uk>
Cc: syzbot <syzbot+01523a0ae5600aef5895@...kaller.appspotmail.com>,
 io-uring@...r.kernel.org, linux-kernel@...r.kernel.org,
 syzkaller-bugs@...glegroups.com, hsweeten@...ionengravers.com,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [syzbot] [io-uring?] KASAN: slab-use-after-free Read in
 io_poll_remove_entries

On 7/22/25 6:47 AM, Ian Abbott wrote:
> On Sun, 20 Jul 2025 13:00:59 -0600
> Jens Axboe <axboe@...nel.dk> 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;
>>  }
>>  
>>  /*
>>
> 
> Thanks for your investigation and initial fix. I think dev->attach_lock
> needs to be write-locked before calling is_device_busy() and released
> after comedi_device_detach() (although that also write-locks it, so we
> need to refactor that). Otherwise, someone could get added to the
> wait_head after is_device_busy() returns.

That's fine too, this is what my v2 addressed as well. I don't know
comedi well enough so I was worried about having ->attach_lock held over
comedi_device_cancel_all() -> do_cancel() -> s->cancel() as that appears
to be a new pattern. If you know this is fine, then yes by all means, we
should just hold ->attach_lock over the whole thing and not need a
detaching state.

-- 
Jens Axboe

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