The Linux kernel vulnerability CVE-2026-64294 concerns improper file ownership checks in the memory management code, specifically in mincore() and madvise(MADV_PAGEOUT) system calls. Since the introduction of idmapped mounts, inode ownership checks were performed against a nop_mnt_idmap, which ignores the file's actual mount idmap. This caused owner_or_capable() checks to fail when the file ownership was translated by an idmap, leading to permission check inconsistencies. The fix involved creating a new file_owner_or_capable() function that correctly applies the mount idmap during ownership checks, replacing the previous incorrect usage.

The vulnerability could cause permission checks to fail unexpectedly in certain scenarios involving idmapped mounts, potentially leading to incorrect access control decisions. However, in typical cases such as mincore() and madvise(MADV_PAGEOUT), the impact is usually benign because other permission checks use the correct idmap. There is no indication of direct exploitation or privilege escalation in the provided data.

A fix has been implemented in the Linux kernel to address this issue by introducing a new function that correctly applies mount idmaps during file ownership checks. Users should update to a Linux kernel version that includes this fix. Patch status is not explicitly confirmed in the provided data; therefore, check the official Linux kernel advisories or vendor updates for the exact fixed versions and apply the official patch accordingly.