Message-ID: <d92489ea-a517-4a8f-a84b-33575379c885@mev.co.uk>
Date: Tue, 22 Jul 2025 14:07:45 +0100
From: Ian Abbott <abbotti@....co.uk>
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, 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 22/07/2025 14:00, Jens Axboe wrote:
> 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.

I think it's fine. do_cancel() (and s->cancel()) do not use attach_lock.

Actually, I was thinking it might be safer just to move the 
`wait_queue_head_t wait_head;` member out of `struct comedi_async` and 
into `struct comedi_subdevice`, then it should not matter if there are 
any transient poll_wait() callers waiting on it.

-- 
-=( Ian Abbott <abbotti@....co.uk> || MEV Ltd. is a company  )=-
-=( registered in England & Wales.  Regd. number: 02862268.  )=-
-=( Regd. addr.: S11 & 12 Building 67, Europa Business Park, )=-
-=( Bird Hall Lane, STOCKPORT, SK3 0XA, UK. || www.mev.co.uk )=-

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