xen/gntalloc: Replace UAPI 1-element array

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@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Cc: xen-devel@lists.xenproject.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20240206170320.work.437-kees@kernel.orgSigned-off-by: Juergen Gross <jgross@suse.com>

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;

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 */
+	union {
 		__u32 gref_ids[1];
+		__DECLARE_FLEX_ARRAY(__u32, gref_ids_flex);
+	};
 };
 
 #define GNTALLOC_FLAG_WRITABLE 1