On Tue, Oct 9, 2018 at 12:38 PM Laurent Vivier [off-list ref] wrote:
This patch allows to have a different binfmt_misc configuration
for each new user namespace. By default, the binfmt_misc configuration
is the one of the previous level, but if the binfmt_misc filesystem is
mounted in the new namespace a new empty binfmt instance is created and
used in this namespace.
For instance, using "unshare" we can start a chroot of an another
architecture and configure the binfmt_misc interpreter without being root
to run the binaries in this chroot.
[...]
quoted hunk ↗ jump to hunk
@@ -823,12 +847,34 @@ static const struct super_operations s_ops = {
static int bm_fill_super(struct super_block *sb, void *data, int silent)
{
int err;
+ struct user_namespace *ns = sb->s_user_ns;
static const struct tree_descr bm_files[] = {
[2] = {"status", &bm_status_operations, S_IWUSR|S_IRUGO},
[3] = {"register", &bm_register_operations, S_IWUSR},
/* last one */ {""}
};
+ /* create a new binfmt namespace
+ * if we are not in the first user namespace
+ * but the binfmt namespace is the first one
+ */
+ if (READ_ONCE(ns->binfmt_ns) == NULL) {
+ struct binfmt_namespace *new_ns;
+
+ new_ns = kmalloc(sizeof(struct binfmt_namespace),
+ GFP_KERNEL);
+ if (new_ns == NULL)
+ return -ENOMEM;
+ INIT_LIST_HEAD(&new_ns->entries);
+ new_ns->enabled = 1;
+ rwlock_init(&new_ns->entries_lock);
+ new_ns->bm_mnt = NULL;
+ new_ns->entry_count = 0;
+ /* ensure new_ns is completely initialized before sharing it */
+ smp_wmb();
+ WRITE_ONCE(ns->binfmt_ns, new_ns);
+ }
You're still not preventing a concurrent race of two mount() calls,
right? What prevents two instances of this code block from running
concurrently in two different namespaces? I think you want to take
some sort of global lock around this.