Xen/gntdev: correct error checking in gntdev_map_grant_pages()

stable inclusion
from linux-4.19.177
commit e07f06f6bbeed5bf47fed79ac6a57ec62b33304a
CVE: CVE-2021-26932

--------------------------------

commit ebee0eab upstream.

Failure of the kernel part of the mapping operation should also be
indicated as an error to the caller, or else it may assume the
respective kernel VA is okay to access.

Furthermore gnttab_map_refs() failing still requires recording
successfully mapped handles, so they can be unmapped subsequently. This
in turn requires there to be a way to tell full hypercall failure from
partial success - preset map_op status fields such that they won't
"happen" to look as if the operation succeeded.

Also again use GNTST_okay instead of implying its value (zero).

This is part of XSA-361.
Signed-off-by: NJan Beulich <jbeulich@suse.com>
Cc: stable@vger.kernel.org
Reviewed-by: NJuergen Gross <jgross@suse.com>
Signed-off-by: NJuergen Gross <jgross@suse.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-by: NXiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Acked-by: NXie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: NCheng Jian <cj.chengjian@huawei.com>

drivers/xen/gntdev.c

@@ -348,21 +348,22 @@ int gntdev_map_grant_pages(struct gntdev_grant_map *map)
 	pr_debug("map %d+%d\n", map->index, map->count);
 	err = gnttab_map_refs(map->map_ops, use_ptemod ? map->kmap_ops : NULL,
 			map->pages, map->count);
-	if (err)
-		return err;
 
 	for (i = 0; i < map->count; i++) {
-		if (map->map_ops[i].status) {
+		if (map->map_ops[i].status == GNTST_okay)
+			map->unmap_ops[i].handle = map->map_ops[i].handle;
+		else if (!err)
 			err = -EINVAL;
-			continue;
-		}
 
 		if (map->flags & GNTMAP_device_map)
 			map->unmap_ops[i].dev_bus_addr = map->map_ops[i].dev_bus_addr;
 
-		map->unmap_ops[i].handle = map->map_ops[i].handle;
-		if (use_ptemod)
+		if (use_ptemod) {
+			if (map->kmap_ops[i].status == GNTST_okay)
 				map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
+			else if (!err)
+				err = -EINVAL;
+		}
 	}
 	return err;
 }

include/xen/grant_table.h

@@ -157,6 +157,7 @@ gnttab_set_map_op(struct gnttab_map_grant_ref *map, phys_addr_t addr,
 	map->flags = flags;
 	map->ref = ref;
 	map->dom = domid;
+	map->status = 1; /* arbitrary positive value */
 }
 
 static inline void