statmount: don't call path_put() under namespace semaphore

Massage statmount() and make sure we don't call path_put() under the
namespace semaphore. If we put the last reference we're fscked.

Fixes: 46eae99 ("add statmount(2) syscall")
Cc: stable@vger.kernel.org # v6.8+
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

fs/namespace.c

@@ -5708,7 +5708,6 @@ static int grab_requested_root(struct mnt_namespace *ns, struct path *root)
 static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
 			struct mnt_namespace *ns)
 {
-	struct path root __free(path_put) = {};
 	struct mount *m;
 	int err;
 
@@ -5720,7 +5719,7 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
 	if (!s->mnt)
 		return -ENOENT;
 
-	err = grab_requested_root(ns, &root);
+	err = grab_requested_root(ns, &s->root);
 	if (err)
 		return err;
 
@@ -5729,16 +5728,14 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
 	 * mounts to show users.
 	 */
 	m = real_mount(s->mnt);
-	if (!is_path_reachable(m, m->mnt.mnt_root, &root) &&
+	if (!is_path_reachable(m, m->mnt.mnt_root, &s->root) &&
 	    !ns_capable_noaudit(ns->user_ns, CAP_SYS_ADMIN))
 		return -EPERM;
 
 	err = security_sb_statfs(s->mnt->mnt_root);
 	if (err)
 		return err;
 
-	s->root = root;
-
 	/*
 	 * Note that mount properties in mnt->mnt_flags, mnt->mnt_idmap
 	 * can change concurrently as we only hold the read-side of the
@@ -5960,6 +5957,7 @@ SYSCALL_DEFINE4(statmount, const struct mnt_id_req __user *, req,
 	if (!ret)
 		ret = copy_statmount_to_user(ks);
 	kvfree(ks->seq.buf);
+	path_put(&ks->root);
 	if (retry_statmount(ret, &seq_size))
 		goto retry;
 	return ret;