CVE-2025-21654

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

ovl: support encoding fid from inode with no alias

Dmitry Safonov reported that a WARN_ON() assertion can be trigered by userspace when calling inotify_show_fdinfo() for an overlayfs watched inode, whose dentry aliases were discarded with drop_caches.

The WARN_ON() assertion in inotify_show_fdinfo() was removed, because it is possible for encoding file handle to fail for other reason, but the impact of failing to encode an overlayfs file handle goes beyond this assertion.

As shown in the LTP test case mentioned in the link below, failure to encode an overlayfs file handle from a non-aliased inode also leads to failure to report an fid with FAN_DELETE_SELF fanotify events.

As Dmitry notes in his analyzis of the problem, ovl_encode_fh() fails if it cannot find an alias for the inode, but this failure can be fixed. ovl_encode_fh() seldom uses the alias and in the case of non-decodable file handles, as is often the case with fanotify fid info, ovl_encode_fh() never needs to use the alias to encode a file handle.

Defer finding an alias until it is actually needed so ovl_encode_fh() will not fail in the common case of FAN_DELETE_SELF fanotify events.

This flaw is rated as Moderate impact because an attacker requires local access to trigger kernel warning assertions or encoding failures, restricting the potential blast radius to local denial of service. The issue arises when generating file identifiers for watched OverlayFS inodes after their associated directory entries have been evicted from the system cache. An unprivileged local user must interact with an active OverlayFS mount and actively monitor filesystem events to reproduce this condition.

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.