CVE-2026-89683: nfsd: fix dentry ref leak on V4ROOT export filehandle lookup

CVE-2026-89683: nfsd: fix dentry ref leak on V4ROOT export filehandle lookup

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

nfsd: fix dentry ref leak on V4ROOT export filehandle lookup

nfsd_set_fh_dentry() leaks the dentry reference from
exportfs_decode_fh_raw() when the NFS3_FHSIZE or NFS_FHSIZE
switch cases detect NFSEXP_V4ROOT and goto out. The out: label
calls exp_put() but never dput(dentry), and fhp->fh_dentry was
never assigned so fh_put() cannot compensate.

A crafted NFSv3 filehandle targeting a V4ROOT export's fsid
triggers the leak on every request.

The Linux kernel CVE team has assigned CVE-2026-89683 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.12 with commit ef7f6c4904d03ccd7478e1ac20ed75f79c4ac444 and fixed in 6.12.109 with commit d493cf57f4c4dae5396cd6ad41e5c209d0615a81
	Issue introduced in 6.12 with commit ef7f6c4904d03ccd7478e1ac20ed75f79c4ac444 and fixed in 6.18.50 with commit c1ae0f973bcba5730871e35976a0ab6878fdac34
	Issue introduced in 6.12 with commit ef7f6c4904d03ccd7478e1ac20ed75f79c4ac444 and fixed in 7.2.4 with commit 11db8df188b34bdb8c6a3a0bf438e5fc8cd75aca
	Issue introduced in 6.12 with commit ef7f6c4904d03ccd7478e1ac20ed75f79c4ac444 and fixed in 7.3-rc1 with commit 6247023fbbec1325029f2d5f2a7cdc0f9f9ea15a

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-89683
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/nfsd/nfsfh.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/d493cf57f4c4dae5396cd6ad41e5c209d0615a81
	https://git.kernel.org/stable/c/c1ae0f973bcba5730871e35976a0ab6878fdac34
	https://git.kernel.org/stable/c/11db8df188b34bdb8c6a3a0bf438e5fc8cd75aca
	https://git.kernel.org/stable/c/6247023fbbec1325029f2d5f2a7cdc0f9f9ea15a