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Message-ID: <CANp29Y4PHQh8Hm-13k_rybECR=+9LvwSiuPPOoJsiv2ZU2EcTw@mail.gmail.com>
Date: Wed, 23 Jul 2025 16:08:34 +0200
From: Aleksandr Nogikh <nogikh@...gle.com>
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
Subject: Re: [syzbot] [io-uring?] KASAN: slab-use-after-free Read in io_poll_remove_entries
On Sun, Jul 20, 2025 at 8:49 PM Jens Axboe <axboe@...nel.dk> 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
FTR now there's, so:
#syz set subsystems: comedi
>
> --
> Jens Axboe
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