CVE-2026-89839 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
f2fs: use the mount idmap for the owner check in f2fs_xattr_advise_set()
f2fs_xattr_advise_set() calls inode_owner_or_capable() with &nop_mnt_idmap before allowing the "system.advise" xattr to be set, instead of the idmap that the VFS passes to the ->set() handler.
f2fs supports idmapped mounts, so on such a mount this checks the caller's fsuid against the unmapped on-disk owner rather than the mapped owner: the actual owner can be wrongly denied with -EPERM and an unrelated caller wrongly allowed. Pass the handler's idmap instead.
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Analysis
by VulDB Data Team • 09/16/2026
The Linux kernel’s F2FS file system contains a privilege escalation vulnerability within its extended attribute handling logic, specifically in the f2fs_xattr_advise_set function. This flaw arises from an incorrect implementation of identity mapping checks when dealing with idmapped mounts. In modern Linux systems, user namespaces allow for unprivileged users to map their local user and group identifiers to different values on a per-mount basis. When a file system supports idmapped mounts, the kernel must translate these mapped identities correctly before performing access control decisions. The vulnerability stems from f2fs_xattr_advise_set invoking inode_owner_or_capable with a static nop_mnt_idmap structure rather than utilizing the specific mount identity map passed by the Virtual File System (VFS) to the set handler. This discrepancy means that the kernel fails to account for the translation of user identifiers, leading to incorrect authorization decisions based on unmapped disk ownership versus mapped runtime identities.
From a technical perspective, this error constitutes an improper access control mechanism where the system checks permissions against the wrong identity context. When a caller attempts to set the system.advise extended attribute, the function should verify if the calling process is authorized under the current mount’s identity mapping rules. By using nop_mnt_idmap, which represents no translation or default root-like behavior in some contexts, the check effectively bypasses the intended isolation boundaries of user namespaces. Consequently, a caller whose mapped identifier does not match the file owner might be incorrectly granted permission if the unmapped on-disk owner happens to align with their real UID, while legitimate owners who are properly mapped may be denied access due to mismatched identifiers resulting in an EPERM error. This breaks the fundamental security model of user namespaces which relies on accurate identity translation for isolation and privilege management.
The operational impact of this vulnerability is significant for environments relying on containerization or multi-tenant setups that utilize Linux user namespaces. Attackers can exploit this flaw to bypass access controls, potentially allowing unauthorized modification of file metadata or attributes that should be restricted to the true owner or privileged processes. Conversely, legitimate administrative actions may fail unexpectedly, causing denial-of-service conditions for valid users who are denied permission due to mapping mismatches. This undermines the integrity and availability guarantees provided by the operating system’s security modules. The vulnerability is classified under CWE-269 Improper Privilege Management as it involves incorrect enforcement of access rights based on identity verification failures. Additionally, this behavior aligns with ATT&CK technique T1078 Valid Accounts, where an attacker leverages valid but misinterpreted credentials or identities to gain unauthorized access to system resources.
Mitigation for this issue requires applying the kernel patch that corrects the idmap usage in f2fs_xattr_advise_set. System administrators should ensure their Linux kernels are updated to versions containing this fix, particularly if they deploy F2FS on systems utilizing user namespaces or containerized workloads. For organizations unable to immediately update kernels, restricting access to extended attributes through mandatory access control policies such as SELinux or AppArmor can provide an additional layer of defense by explicitly defining which users and processes are permitted to modify system.advise xattrs regardless of the underlying file ownership checks. Regular auditing of kernel updates and monitoring for unexpected permission denials in logs related to F2FS mounts can also help detect potential exploitation attempts or misconfigurations arising from this flaw until a permanent patch is applied.