The Linux kernel vulnerability CVE-2026-74595 concerns the fscrypt_ioctl_set_policy() function in the fscrypt subsystem. This function previously called inode_owner_or_capable() with a fixed mount idmap (&nop_mnt_idmap) rather than the idmap of the mount on which the ioctl was issued. Since fscrypt is used by filesystems supporting idmapped mounts (e.g., ext4, f2fs), this caused the caller's filesystem user ID (fsuid) to be compared against the unmapped on-disk owner instead of the mapped owner. Consequently, the actual owner could be incorrectly denied access (-EACCES), and unrelated callers could be incorrectly allowed to set encryption policies. The vulnerability was resolved by changing the code to use file_mnt_idmap(filp), which correctly references the mount idmap of the file involved in the ioctl call.

This vulnerability could lead to improper access control enforcement in the fscrypt encryption policy setting process on affected filesystems. Legitimate owners might be denied permission to set encryption policies, while unauthorized users might be granted such permissions. This could affect filesystem security and data protection on systems using idmapped mounts. However, there is no indication of known exploits in the wild.

A fix for this vulnerability has been implemented in the Linux kernel by correcting the mount idmap used in the owner check within fscrypt_ioctl_set_policy(). Users should apply the official kernel updates that include this fix. Since the vulnerability has been resolved, applying the updated kernel version is the recommended remediation. Patch status is not explicitly confirmed in the provided data; users should consult the Linux kernel vendor advisory or distribution security notices for the exact fixed versions and update instructions.