Re: [PATCH v3] vfio/platform: prevent read-only region mappings from becoming writable

[Pranjal Shrivastava <praan@google.com>]

On Thu, Sep 03, 2026 at 12:29:07PM +0300, Abdifatah Suruur wrote:
> vfio_platform_mmap() rejects writable mappings of regions without the
> WRITE flag, but leaves VM_MAYWRITE set.  Userspace can map such a region
> read-only and then upgrade the mapping to writable with mprotect().
> 
> Clear VM_MAYWRITE for regions without the WRITE flag, as i915 does for
> its read-only objects and as fixed in drm/vc4 (CVE-2026-68445),
> drm/panthor (CVE-2024-53071) and commit a5edadbae57e ("ptp: vmclock:
> prevent read-only mappings from becoming writable").
> 
> Note this is defensive hardening: no in-tree platform driver currently
> publishes a region without the WRITE flag.  The guard costs nothing and
> keeps the mmap() interface honest if a read-only region ever appears.
> 
> Fixes: fad4d5b1f042 ("vfio/platform: support MMAP of MMIO regions")
> Signed-off-by: Abdifatah Suruur <suruurism@gmail.com>
> Reviewed-by: Mostafa Saleh <smostafa@google.com>
> ---
> v2: commit log now states plainly that the read-only path is
> unreachable in practice, per Alex's review; dropped Cc: stable as
> there is no reachable issue to backport.
> v3: use 'commit <sha> ("<title>")' style for the vmclock reference
> and point Fixes at the MMAP support commit, per Mostafa's review.
> 
> --- a/drivers/vfio/platform/vfio_platform_common.c
> +++ b/drivers/vfio/platform/vfio_platform_common.c
> @@ -594,7 +594,11 @@
>  	if (!(vdev->regions[index].flags & VFIO_REGION_INFO_FLAG_WRITE)
>  			&& (vma->vm_flags & VM_WRITE))
>  		return -EINVAL;
> 
> +	/* Prevent read-only region mappings from being upgraded with mprotect() */
> +	if (!(vdev->regions[index].flags & VFIO_REGION_INFO_FLAG_WRITE))
> +		vm_flags_clear(vma, VM_MAYWRITE);
> +

Minor Nit: We seem to be evaluating this condition twice, back-to-back:

if (!(vdev->regions[index].flags & VFIO_REGION_INFO_FLAG_WRITE))

We could consolidate this into a single block to make the intent and
flow cleaner:

        /* Prevent read-only region mappings from [...] */
        if (!(vdev->regions[index].flags & VFIO_REGION_INFO_FLAG_WRITE)) {
            if (vma->vm_flags & VM_WRITE)
                return -EINVAL;
            vm_flags_clear(vma, VM_MAYWRITE);
        }

Apart from that,

Reviewed-by: Pranjal Shrivastava <praan@google.com>

Thanks,
Praan