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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> <20250722134724.6671e45b@ian-deb> Content-Language: en-US From: Jens Axboe In-Reply-To: <20250722134724.6671e45b@ian-deb> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/22/25 6:47 AM, Ian Abbott wrote: > On Sun, 20 Jul 2025 13:00:59 -0600 > 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; >> } >> >> /* >> > > 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