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.