Re: [PATCH v2] comedi: pcl818: fix null-ptr-deref in pcl818_ai_cancel()

[Ian Abbott]

On 23/10/2025 15:14, Nikita Zhandarovich wrote:
> Syzbot identified an issue [1] in pcl818_ai_cancel(), which stems from
> the fact that in case of early device detach via pcl818_detach(),
> subdevice dev->read_subdev may not have initialized its pointer to
> &struct comedi_async as intended. Thus, any such dereferencing of
> &s->async->cmd will lead to general protection fault and kernel crash.
>
> Mitigate this problem by removing a call to pcl818_ai_cancel() from
> pcl818_detach() altogether. This way, if the subdevice setups its
> support for async commands, everything async-related will be
> handled via subdevice's own ->cancel() function in
> comedi_device_detach_locked() even before pcl818_detach(). If no
> support for asynchronous commands is provided, there is no need
> to cancel anything either.
>
> [1] Syzbot crash:
> Oops: general protection fault, probably for non-canonical address 0xdffffc0000000005: 0000 [#1] SMP KASAN PTI
> KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
> CPU: 1 UID: 0 PID: 6050 Comm: syz.0.18 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
> RIP: 0010:pcl818_ai_cancel+0x69/0x3f0 drivers/comedi/drivers/pcl818.c:762
> ...
> Call Trace:
>   <TASK>
>   pcl818_detach+0x66/0xd0 drivers/comedi/drivers/pcl818.c:1115
>   comedi_device_detach_locked+0x178/0x750 drivers/comedi/drivers.c:207
>   do_devconfig_ioctl drivers/comedi/comedi_fops.c:848 [inline]
>   comedi_unlocked_ioctl+0xcde/0x1020 drivers/comedi/comedi_fops.c:2178
>   vfs_ioctl fs/ioctl.c:51 [inline]
>   __do_sys_ioctl fs/ioctl.c:597 [inline]
> ...
>
> Reported-by: syzbot+fce5d9d5bd067d6fbe9b@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=fce5d9d5bd067d6fbe9b
> Fixes: 00aba6e7b565 ("staging: comedi: pcl818: remove 'neverending_ai' from private data")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Nikita Zhandarovich <n.zhandarovich@xxxxxxxxxx>
> ---
> v1 -> v2: Switch to a better, more logical approach, put forward by
> Ian Abbott <abbotti@xxxxxxxxx>, instead of doing an awkward
> check in pcl818_ai_cancel() itself. Adjust commit description.
>
> P.S. I've chosen to keep the old Fixes: tag. Not sure it's best,
> but it's at least partially correct.
>
>   drivers/comedi/drivers/pcl818.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/comedi/drivers/pcl818.c b/drivers/comedi/drivers/pcl818.c
> index 4127adcfb229..06fe06396f23 100644
> --- a/drivers/comedi/drivers/pcl818.c
> +++ b/drivers/comedi/drivers/pcl818.c
> @@ -1111,10 +1111,9 @@ static void pcl818_detach(struct comedi_device *dev)
>   {
>   	struct pcl818_private *devpriv = dev->private;
>   
> -	if (devpriv) {
> -		pcl818_ai_cancel(dev, dev->read_subdev);
> +	if (devpriv)
>   		pcl818_reset(dev);
> -	}
> +
>   	pcl818_free_dma(dev);
>   	comedi_legacy_detach(dev);
>   }

Looks good. Thanks for the patch!

Reviewed-by: Ian Abbott <abbotti@xxxxxxxxx>

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