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.

This vulnerability is rated as Moderate severity because exploitation requires local access and complex race condition timing to trigger a use-after-free condition, rather than enabling remote compromise. The flaw stems from mount ID-mapping operations on detached mounts within newer upstream virtual filesystem interfaces. Red Hat Enterprise Linux kernels are not affected because the vulnerable code introducing this capability was never backported or included in supported product releases.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.