Message-ID: <404ab8cf-7a3a-4fc1-ab99-28c9b3f0b8c6@kernel.dk>
Date: Tue, 22 Jul 2025 06:43:55 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Ian Abbott <abbotti@....co.uk>,
 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 6:30 AM, Ian Abbott wrote:
> On 22/07/2025 13:21, 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?
>>
> 
> Not yet, please. I'm working on it to close the remaining part of the race condition window.

I updated mine, see below. Will fling it at syzbot shortly.

diff --git a/drivers/comedi/comedi_fops.c b/drivers/comedi/comedi_fops.c
index c83fd14dd7ad..58b034e45283 100644
--- a/drivers/comedi/comedi_fops.c
+++ b/drivers/comedi/comedi_fops.c
@@ -782,24 +782,33 @@ void comedi_device_cancel_all(struct comedi_device *dev)
 	}
 }
 
-static int is_device_busy(struct comedi_device *dev)
+static int start_detach(struct comedi_device *dev)
 {
 	struct comedi_subdevice *s;
-	int i;
+	int i, is_busy = 0;
 
 	lockdep_assert_held(&dev->mutex);
+	lockdep_assert_held(&dev->attach_lock);
 	if (!dev->attached)
 		return 0;
 
 	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) ||
+		    wq_has_sleeper(&s->async->wait_head)) {
+			is_busy = 1;
+			break;
+		}
 	}
-
-	return 0;
+	if (!is_busy)
+		dev->detaching = 1;
+	return is_busy;
 }
 
 /*
@@ -825,8 +834,13 @@ static int do_devconfig_ioctl(struct comedi_device *dev,
 		return -EPERM;
 
 	if (!arg) {
-		if (is_device_busy(dev))
+		/* prevent new polls */
+		down_write(&dev->attach_lock);
+		if (start_detach(dev)) {
+			up_write(&dev->attach_lock);
 			return -EBUSY;
+		}
+		up_write(&dev->attach_lock);
 		if (dev->attached) {
 			struct module *driver_module = dev->driver->module;
 
@@ -2479,7 +2493,7 @@ static __poll_t comedi_poll(struct file *file, poll_table *wait)
 
 	down_read(&dev->attach_lock);
 
-	if (!dev->attached) {
+	if (!dev->attached || dev->detaching) {
 		dev_dbg(dev->class_dev, "no driver attached\n");
 		goto done;
 	}
diff --git a/include/linux/comedi/comedidev.h b/include/linux/comedi/comedidev.h
index 4cb0400ad616..b2bec668785f 100644
--- a/include/linux/comedi/comedidev.h
+++ b/include/linux/comedi/comedidev.h
@@ -545,6 +545,7 @@ struct comedi_device {
 	const char *board_name;
 	const void *board_ptr;
 	unsigned int attached:1;
+	unsigned int detaching:1;
 	unsigned int ioenabled:1;
 	spinlock_t spinlock;	/* generic spin-lock for low-level driver */
 	struct mutex mutex;	/* generic mutex for COMEDI core */


-- 
Jens Axboe

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