CVE-2026-89711NFSD: remove flawed WARN_ON_ONCE from nfsd_mode_check

Official 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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/ae251937c6f0237e5b555a5e4ba4595b8206e865
• https://git.kernel.org/stable/c/e145e8d67a5d41c72d322e7f87ab474d39d9dfb7
• https://git.kernel.org/stable/c/9f4434893a2783f7384d993cea883aff3fb7a52d
• https://git.kernel.org/stable/c/2ef131323999539038e306773c8256403834361b
• https://git.kernel.org/stable/c/b55b4d880bb080fa10eb08ba21a5d8679b8102fe
• https://git.kernel.org/stable/c/7ef182a8fe9c12b0d936880b1e504840639aa009
• https://git.kernel.org/stable/c/a275de3bac5635514ca830f2e46b5ff0e66b5c4c
• https://git.kernel.org/stable/c/aa0cf48a448c5a9fe1a1e880899ecd589ce39e6e