[PATCH] xen/gntalloc: Replace UAPI 1-element array

[Kees Cook <keescook@xxxxxxxxxxxx>]

Without changing the structure size (since it is UAPI), add a proper
flexible array member, and reference it in the kernel so that it will
not be trip the array-bounds sanitizer[1].

Link: https://github.com/KSPP/linux/issues/113 [1]
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
Cc: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/xen/gntalloc.c      | 2 +-
 include/uapi/xen/gntalloc.h | 5 ++++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/gntalloc.c b/drivers/xen/gntalloc.c
index 26ffb8755ffb..f93f73ecefee 100644
--- a/drivers/xen/gntalloc.c
+++ b/drivers/xen/gntalloc.c
@@ -317,7 +317,7 @@ static long gntalloc_ioctl_alloc(struct 
gntalloc_file_private_data *priv,
                rc = -EFAULT;
                goto out_free;
        }
-       if (copy_to_user(arg->gref_ids, gref_ids,
+       if (copy_to_user(arg->gref_ids_flex, gref_ids,
                        sizeof(gref_ids[0]) * op.count)) {
                rc = -EFAULT;
                goto out_free;
diff --git a/include/uapi/xen/gntalloc.h b/include/uapi/xen/gntalloc.h
index 48d2790ef928..3109282672f3 100644
--- a/include/uapi/xen/gntalloc.h
+++ b/include/uapi/xen/gntalloc.h
@@ -31,7 +31,10 @@ struct ioctl_gntalloc_alloc_gref {
        __u64 index;
        /* The grant references of the newly created grant, one per page */
        /* Variable size, depending on count */
-       __u32 gref_ids[1];
+       union {
+               __u32 gref_ids[1];
+               __DECLARE_FLEX_ARRAY(__u32, gref_ids_flex);
+       };
 };
 
 #define GNTALLOC_FLAG_WRITABLE 1
-- 
2.34.1