[PATCH 2/3] virtio_ring: check used_event offset

["Michael S. Tsirkin" <mst@redhat.com>]

Nothing's wrong with vring_size as is, but it's nice
to check that the new field in the avail ring
won't overlow into the used ring.

Reported-by: Tom Lendacky <tahm@linux.vnet.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/linux/virtio_ring.h |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index 2a3b0ea..089cbf2 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -119,7 +119,13 @@ static inline void vring_init(struct vring *vr, unsigned int num, void *p,
 
 static inline unsigned vring_size(unsigned int num, unsigned long align)
 {
-	return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (2 + num)
+#ifdef __KERNEL__
+	/* Older versions did not have used_event field at the end of the
+	 * avail ring. Used ring offset must be compatible with such devices. */
+	size_t s = sizeof(struct vring_desc) * num + sizeof(__u16) * (2 + num);
+	BUG_ON(ALIGN(s, align) != ALIGN(s + sizeof(__u16), align));
+#endif
+	return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (3 + num)
 		 + align - 1) & ~(align - 1))
 		+ sizeof(__u16) * 3 + sizeof(struct vring_used_elem) * num;
 }
-- 
1.7.5.53.gc233e