vfio: align capability structures

The VFIO_DEVICE_GET_INFO, VFIO_DEVICE_GET_REGION_INFO, and
VFIO_IOMMU_GET_INFO ioctls fill in an info struct followed by capability
structs:

  +------+---------+---------+-----+
  | info | caps[0] | caps[1] | ... |
  +------+---------+---------+-----+

Both the info and capability struct sizes are not always multiples of
sizeof(u64), leaving u64 fields in later capability structs misaligned.

Userspace applications currently need to handle misalignment manually in
order to support CPU architectures and programming languages with strict
alignment requirements.

Make life easier for userspace by ensuring alignment in the kernel. This
is done by padding info struct definitions and by copying out zeroes
after capability structs that are not aligned.

The new layout is as follows:

  +------+---------+---+---------+-----+
  | info | caps[0] | 0 | caps[1] | ... |
  +------+---------+---+---------+-----+

In this example caps[0] has a size that is not multiples of sizeof(u64),
so zero padding is added to align the subsequent structure.

Adding zero padding between structs does not break the uapi. The memory
layout is specified by the info.cap_offset and caps[i].next fields
filled in by the kernel. Applications use these field values to locate
structs and are therefore unaffected by the addition of zero padding.

Note that code that copies out info structs with padding is updated to
always zero the struct and copy out as many bytes as userspace
requested. This makes the code shorter and avoids potential information
leaks by ensuring padding is initialized.

Originally-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20230809203144.2880050-1-stefanha@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
index fa06e3eb4955..f9c6f3e2cf6e 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;
 };
 #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;
 };
 
 /*