CVE-2025-40212

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

nfsd: fix refcount leak in nfsdsetfh_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, nfsdsetfhdentry() will report an error, but still stores the export in "struct svcfh" even though it also drops the reference (expput()). This means that when fhput() 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.