CVE-2025-39717

In the Linux kernel, the following vulnerability has been resolved:

open_tree_attr: do not allow id-mapping changes without OPEN_TREE_CLONE

As described in commit 7a54947e727b ('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 2462651ffa76 ("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.