open_tree_attr: do not allow id-mapping changes without OPEN_TREE_CLONE

As described in commit 7a54947 ('Merge patch series "fs: allow
changing idmappings"'), open_tree_attr(2) was necessary in order to
allow for a detached mount to be created and have its idmappings changed
without the risk of any racing threads operating on it. For this reason,
mount_setattr(2) still does not allow for id-mappings to be changed.

However, there was a bug in commit 2462651 ("fs: allow changing
idmappings") which allowed users to bypass this restriction by calling
open_tree_attr(2) *without* OPEN_TREE_CLONE.

can_idmap_mount() prevented this bug from allowing an attached
mountpoint's id-mapping from being modified (thanks to an is_anon_ns()
check), but this still allows for detached (but visible) mounts to have
their be id-mapping changed. This risks the same UAF and locking issues
as described in the merge commit, and was likely unintentional.

Fixes: 2462651 ("fs: allow changing idmappings")
Cc: stable@vger.kernel.org # v6.15+
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/20250808-open_tree_attr-bugfix-idmap-v1-1-0ec7bc05646c@cyphar.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: cyphar

fs/namespace.c

@@ -5176,7 +5176,8 @@ SYSCALL_DEFINE5(open_tree_attr, int, dfd, const char __user *, filename,
 		int ret;
 		struct mount_kattr kattr = {};
 
-		kattr.kflags = MOUNT_KATTR_IDMAP_REPLACE;
+		if (flags & OPEN_TREE_CLONE)
+			kattr.kflags = MOUNT_KATTR_IDMAP_REPLACE;
 		if (flags & AT_RECURSIVE)
 			kattr.kflags |= MOUNT_KATTR_RECURSE;