CVE-2026-89711

/// Technical description

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.

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References