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Message-ID: <d407c9f1-e625-4153-930f-6e44d82b32b5@kernel.dk>
Date: Sun, 20 Jul 2025 13:00:59 -0600
From: Jens Axboe <axboe@...nel.dk>
To: 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, Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [syzbot] [io-uring?] KASAN: slab-use-after-free Read in
io_poll_remove_entries
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;
}
/*
--
Jens Axboe
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