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io_poll_remove_entries To: Ian Abbott , syzbot , io-uring@vger.kernel.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, hsweeten@visionengravers.com, Greg Kroah-Hartman References: <687bd5fe.a70a0220.693ce.0091.GAE@google.com> <9385a1a6-8c10-4eb5-9ab9-87aaeb6a7766@kernel.dk> <19daff9b-9b57-406b-8ee5-1d4ef5ba2855@mev.co.uk> Content-Language: en-US From: Jens Axboe In-Reply-To: <19daff9b-9b57-406b-8ee5-1d4ef5ba2855@mev.co.uk> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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@syzkaller.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: >>>>>> >>>>>> __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 >>>>>> >>>>>> >>>>>> 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