CVE-2025-40212

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

nfsd: fix refcount leak in nfsd_set_fh_dentry()

nfsd exports a "pseudo root filesystem" which is used by NFSv4 to find the various exported filesystems using LOOKUP requests from a known root filehandle. NFSv3 uses the MOUNT protocol to find those exported filesystems and so is not given access to the pseudo root filesystem.

If a v3 (or v2) client uses a filehandle from that filesystem, nfsd_set_fh_dentry() will report an error, but still stores the export in "struct svc_fh" even though it also drops the reference (exp_put()). This means that when fh_put() is called an extra reference will be dropped which can lead to use-after-free and possible denial of service.

Normal NFS usage will not provide a pseudo-root filehandle to a v3 client. This bug can only be triggered by the client synthesising an incorrect filehandle.

To fix this we move the assignments to the svc_fh later, after all possible error cases have been detected.

A refcount imbalance in nfsd_set_fh_dentry() could lead to a use-after-free when processing a crafted NFS filehandle that references the NFSv4 pseudo-root from an NFSv3 request. A remote attacker able to send malformed filehandles to an NFS server could trigger a kernel crash (DoS).

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.