CVE-2026-89711 in Linux
Summary
by MITRE • 09/11/2026
In the Linux kernel, the following vulnerability has been resolved:
NFSD: remove flawed WARN_ON_ONCE from nfsd_mode_check
The header for commit e75b23f9e323 ("nfsd: check d_can_lookup in fh_verify of directories") details the assumption that justified adding the WARN_ON_ONCE to nfsd_mode_check(), that assumption is invalid (in the case of NFS reexport).
When NFSD exports an NFS filesystem it is very possible for nfsd_mode_check() to encounter a @dentry that doesn't have i_op->lookup (see nfs_fhget()'s NFS_ATTR_FATTR_MOUNTPOINT and NFS_ATTR_FATTR_V4_REFERRAL handling, and d_flags_for_inode()).
So remove nfsd_mode_check()'s WARN_ON_ONCE(). The nfserr_notdir return on that branch must stay. It guards the subsequent lookup_one_unlocked() -> __lookup_slow() path, which calls inode->i_op->lookup() with no NULL check, so returning nfserr_notdir is what keeps a client LOOKUP into such a @dentry from dereferencing a NULL method pointer.
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Analysis
by VulDB Data Team • 09/11/2026
The vulnerability addressed in this update pertains to the Network File System server implementation within the Linux kernel, specifically involving the function nfsd_mode_check located in the fs/nfsd/vfs.c module. This issue stems from an incorrect assumption made during the development of a previous patch that introduced a WARN_ON_ONCE macro into the code path responsible for verifying file handle access modes. The original intent was to catch unexpected states where a directory entry did not support standard lookup operations, but this safety check proved to be overly aggressive and technically flawed in specific operational contexts involving NFS re-export scenarios.
In environments where an NFS server exports another NFS filesystem, known as NFS reexporting, the kernel encounters dentry objects that may lack the i_op->lookup method pointer entirely. This situation arises during the handling of mount point attributes or version four referrals, where the inode structure is configured differently than in standard local file system operations. The presence of a WARN_ON_ONCE macro triggers when such an absence is detected, leading to kernel warnings and potential log spam that can obscure genuine security alerts or cause performance degradation due to excessive logging overhead. More critically, if not handled correctly, this could lead to instability as the code path assumes the existence of lookup methods for all directory entries.
The core technical flaw lies in the assumption that every dentry passed through nfsd_mode_check will have a valid inode operation structure containing a lookup function pointer. However, NFS reexport configurations explicitly allow for dentries representing remote mounts or referrals where this method is intentionally absent because the resolution logic differs from local filesystem lookups. By retaining the WARN_ON_ONCE check, the kernel incorrectly flags these legitimate states as anomalies, which violates the principle of robust error handling and can lead to unnecessary alerting mechanisms being triggered during normal administrative operations involving nested NFS exports.
To mitigate this issue, the flawed WARN_ON_ONCE macro was removed from nfsd_mode_check while preserving the critical security guard that returns an nfserr_notdir error code when a lookup operation is attempted on such dentries. This retention of the error return value is vital because it prevents subsequent functions like lookup_one_unlocked and __lookup_slow from attempting to dereference a NULL method pointer, which would otherwise result in a kernel panic or undefined behavior due to invalid memory access. The fix ensures that clients attempting LOOKUP operations against these specific dentry types receive an appropriate protocol-level error rather than causing a system crash.
From a vulnerability classification perspective, this issue relates to CWE-673, External Influence of Untrusted Variables, as the kernel must correctly handle inputs derived from external NFS client requests without assuming internal state consistency that does not hold in all export configurations. It also touches upon aspects of robustness and error handling often associated with CWE-252, unchecked return value, although here it is more about incorrect precondition checking leading to potential instability. In the context of MITRE ATT&CK, this falls under T1068 Exploitation for Privilege Escalation if an attacker could trigger the warning condition repeatedly to cause denial of service through log flooding or system instability, though primarily it represents a stability and correctness fix rather than a direct exploitation vector.
The resolution emphasizes the importance of aligning kernel assumptions with actual protocol behaviors in complex networking scenarios like NFS reexporting. By removing the erroneous assertion while maintaining strict error checking for NULL pointer dereferences, the Linux kernel maintains both operational integrity and security posture. Administrators managing multi-layered NFS deployments should ensure their kernels are updated to this corrected version to prevent spurious warnings and potential instability during routine file system operations involving remote mounts or referrals.