Re: [PATCH] vfio: align capability structures

Jason Gunthorpe
2023-08-08T23:29:30Z

On Thu, Aug 03, 2023 at 03:18:23PM -0600, Alex Williamson wrote:

> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 902f06e52c48..2d074cbd371d 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -1362,6 +1362,8 @@ struct vfio_info_cap_header *vfio_info_cap_add(struct vfio_info_cap *caps,
> void *buf;
> struct vfio_info_cap_header *header, *tmp;
>
> + size = ALIGN(size, sizeof(u64));
> +
> buf = krealloc(caps->buf, caps->size + size, GFP_KERNEL);
> if (!buf) {
> kfree(caps->buf);
> @@ -1395,6 +1397,8 @@ void vfio_info_cap_shift(struct vfio_info_cap *caps, size_t offset)
> struct vfio_info_cap_header *tmp;
> void *buf = (void *)caps->buf;
>
> + WARN_ON(!IS_ALIGNED(offset, sizeof(u64)));
> +
> for (tmp = buf; tmp->next; tmp = buf + tmp->next - offset)
> tmp->next += offset;
> }
> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> index fa06e3eb4955..fd2761841ffe 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -217,6 +217,7 @@ struct vfio_device_info {
> __u32 num_regions; /* Max region index + 1 */
> __u32 num_irqs; /* Max IRQ index + 1 */
> __u32 cap_offset; /* Offset within info struct of first cap */
> + __u32 pad; /* Size must be aligned for caps */
> };
> #define VFIO_DEVICE_GET_INFO _IO(VFIO_TYPE, VFIO_BASE + 7)
>
> @@ -1444,6 +1445,7 @@ struct vfio_iommu_type1_info {
> #define VFIO_IOMMU_INFO_CAPS (1 << 1) /* Info supports caps */
> __u64 iova_pgsizes; /* Bitmap of supported page sizes */
> __u32 cap_offset; /* Offset within info struct of first cap */
> + __u32 pad; /* Size must be aligned for caps */
> };

IMHO this is partially being caused by not using __aligned_u64 for the
other __u64's in the same struct..

Both of these structs have u64s in them and many arches will
automatically add the above padding. __aligned_u64 will force the
reset to do it, and then making padding explicit as you have done will
make it really true.

This is a subtle x64/x32 compatability issue also. It is probably best
just to do the change across the whole header file.

Please also include the matching hunk for iommufd:

--- a/drivers/iommu/iommufd/vfio_compat.c
+++ b/drivers/iommu/iommufd/vfio_compat.c
@@ -483,6 +483,8 @@ static int iommufd_vfio_iommu_get_info(struct iommufd_ctx *ictx,
rc = cap_size;
goto out_put;
}
+ cap_size = ALIGN(cap_size, sizeof(u64));
+
if (last_cap && info.argsz >= total_cap_size &&
put_user(total_cap_size, &last_cap->next)) {
rc = -EFAULT;

Thanks,
Jason