Message-ID: <20250722134724.6671e45b@ian-deb>
Date: Tue, 22 Jul 2025 13:47:24 +0100
From: Ian Abbott <abbotti@....co.uk>
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, 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 Sun, 20 Jul 2025 13:00:59 -0600
Jens Axboe <axboe@...nel.dk> 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;
>  }
>  
>  /*
> 

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.

Something like this:

diff --git a/drivers/comedi/comedi_fops.c b/drivers/comedi/comedi_fops.c
index 3383a7ce27ff..a0048510fc40 100644
--- a/drivers/comedi/comedi_fops.c
+++ b/drivers/comedi/comedi_fops.c
@@ -787,6 +787,7 @@ static int is_device_busy(struct comedi_device *dev)
 	struct comedi_subdevice *s;
 	int i;
 
+	lockdep_assert_held_write(&dev->attach_lock);
 	lockdep_assert_held(&dev->mutex);
 	if (!dev->attached)
 		return 0;
@@ -795,7 +796,12 @@ static int is_device_busy(struct comedi_device *dev)
 		s = &dev->subdevices[i];
 		if (s->busy)
 			return 1;
-		if (s->async && comedi_buf_is_mmapped(s))
+		if (!s->async)
+			continue;
+		if (comedi_buf_is_mmapped(s))
+			return 1;
+		/* There may be transient active polls. */
+		if (waitqueue_active(&s->async->wait_head))
 			return 1;
 	}
 
@@ -825,15 +831,22 @@ static int do_devconfig_ioctl(struct comedi_device *dev,
 		return -EPERM;
 
 	if (!arg) {
-		if (is_device_busy(dev))
-			return -EBUSY;
+		int rc = 0;
+
 		if (dev->attached) {
-			struct module *driver_module = dev->driver->module;
+			down_write(&dev->attach_lock);
+			if (is_device_busy(dev)) {
+				rc = -EBUSY;
+			} else {
+				struct module *driver_module =
+					dev->driver->module;
 
-			comedi_device_detach(dev);
-			module_put(driver_module);
+				comedi_device_detach_locked(dev);
+				module_put(driver_module);
+			}
+			up_write(&dev->attach_lock);
 		}
-		return 0;
+		return rc;
 	}
 
 	if (copy_from_user(&it, arg, sizeof(it)))
diff --git a/drivers/comedi/comedi_internal.h b/drivers/comedi/comedi_internal.h
index 9b3631a654c8..cf10ba016ebc 100644
--- a/drivers/comedi/comedi_internal.h
+++ b/drivers/comedi/comedi_internal.h
@@ -50,6 +50,7 @@ extern struct mutex comedi_drivers_list_lock;
 int insn_inval(struct comedi_device *dev, struct comedi_subdevice *s,
 	       struct comedi_insn *insn, unsigned int *data);
 
+void comedi_device_detach_locked(struct comedi_device *dev);
 void comedi_device_detach(struct comedi_device *dev);
 int comedi_device_attach(struct comedi_device *dev,
 			 struct comedi_devconfig *it);
diff --git a/drivers/comedi/drivers.c b/drivers/comedi/drivers.c
index 376130bfba8a..f5c2ee271d0e 100644
--- a/drivers/comedi/drivers.c
+++ b/drivers/comedi/drivers.c
@@ -158,7 +158,7 @@ static void comedi_device_detach_cleanup(struct comedi_device *dev)
 	int i;
 	struct comedi_subdevice *s;
 
-	lockdep_assert_held(&dev->attach_lock);
+	lockdep_assert_held_write(&dev->attach_lock);
 	lockdep_assert_held(&dev->mutex);
 	if (dev->subdevices) {
 		for (i = 0; i < dev->n_subdevices; i++) {
@@ -196,16 +196,23 @@ static void comedi_device_detach_cleanup(struct comedi_device *dev)
 	comedi_clear_hw_dev(dev);
 }
 
-void comedi_device_detach(struct comedi_device *dev)
+void comedi_device_detach_locked(struct comedi_device *dev)
 {
+	lockdep_assert_held_write(&dev->attach_lock);
 	lockdep_assert_held(&dev->mutex);
 	comedi_device_cancel_all(dev);
-	down_write(&dev->attach_lock);
 	dev->attached = false;
 	dev->detach_count++;
 	if (dev->driver)
 		dev->driver->detach(dev);
 	comedi_device_detach_cleanup(dev);
+}
+
+void comedi_device_detach(struct comedi_device *dev)
+{
+	lockdep_assert_held(&dev->mutex);
+	down_write(&dev->attach_lock);
+	comedi_device_detach_locked(dev);
 	up_write(&dev->attach_lock);
 }

-- 
-=( Ian Abbott <abbotti@....co.uk> || MEV Ltd. is a company  )=-
-=( registered in England & Wales.  Regd. number: 02862268.  )=-
-=( Regd. addr.: S11 & 12 Building 67, Europa Business Park, )=-
-=( Bird Hall Lane, STOCKPORT, SK3 0XA, UK. || www.mev.co.uk )=-

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