Re: [PATCH v9 4/4] vfio-pci/zdev: Add VFIO FMB device features

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] vfio_pci: NULL pointer deref in fmb_read on fmb allocation failure
- [High] vfio_pci: stack buffer overflow in fmb_read via unbounded fmb_length
--

commit 759143700fb3898fd78661e48f994eff036f8cf5
Author: Omar Elghoul <oelghoul@linux.ibm.com>

vfio-pci/zdev: Add VFIO FMB device features

This commit introduces new VFIO features for zPCI devices to provide FMB
passthrough to userspace. It allows enabling or disabling the FMB using a SET
feature, and reading the latest FMB using a GET feature.

> diff --git a/drivers/vfio/pci/vfio_pci_zdev.c b/drivers/vfio/pci/vfio_pci_zdev.c
> index f47f36314a1cc..1a66f455545ab 100644
> --- a/drivers/vfio/pci/vfio_pci_zdev.c
> +++ b/drivers/vfio/pci/vfio_pci_zdev.c
[ ... ]
> +int vfio_pci_zdev_feature_fmb_enable(struct vfio_pci_core_device *vdev, u32 flags,
> +				     void __user *arg, size_t argsz)
> +{
[ ... ]
> +	if (fmb_enable.enabled) {
> +		zdev->fmb_requested = true;
> +		return zpci_fmb_reenable_device(zdev);

[Severity: High]
If zpci_fmb_reenable_device() fails (for example, if kmem_cache_zalloc()
returns -ENOMEM), does zdev->fmb_requested remain set to true?

Since the failure doesn't revert the flag to false, could this leave the
device state inconsistent for future FMB reads?

[Severity: High]
Is it intentional to bypass the zdev->fmb_length bounds check here?

The standard enablement path typically validates that zdev->fmb_length fits
within the structure boundaries. By calling zpci_fmb_reenable_device()
directly, it appears a hardware-supplied length greater than the size of
struct zpci_fmb could be accepted and saved.

> +	}
> +
> +	return zpci_fmb_disable_device(zdev);
> +}
> +
> +int vfio_pci_zdev_feature_fmb_read(struct vfio_pci_core_device *vdev, u32 flags,
> +				   void __user *arg, size_t argsz)
> +{
> +	struct zpci_dev *zdev;
> +	struct vfio_device_feature_zpci_fmb_read fmb_read;
> +	struct zpci_fmb fmb_bounce;
[ ... ]
> +	scoped_guard(mutex, &zdev->fmb_lock) {
> +		if (!zdev->fmb_requested)
> +			return -ENOMSG;
> +		memcpy(&fmb_bounce, zdev->fmb, zdev->fmb_length);

[Severity: High]
If a previous call to vfio_pci_zdev_feature_fmb_enable() failed during
allocation but left zdev->fmb_requested = true, this check will pass.

Will this result in a NULL pointer dereference when accessing zdev->fmb in
this memcpy()?

[Severity: High]
Can zdev->fmb_length be larger than sizeof(struct zpci_fmb)?

If an oversized fmb_length is reported by the hardware or firmware and was not
validated during enablement, could copying up to that length into the
fixed-size local stack variable fmb_bounce result in a stack buffer overflow?

> +	}
> +
> +	if (copy_to_user(u64_to_user_ptr(fmb_read.data), &fmb_bounce, zdev->fmb_length))
> +		return -EFAULT;
> +	return 0;
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261007135926.82935-1-oelghoul@linux.ibm.com?part=4