CVE-2026-89696: nfsd: block non-SAVEFH ops after FOREIGN PUTFH to prevent NULL deref

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

nfsd: block non-SAVEFH ops after FOREIGN PUTFH to prevent NULL deref

When CONFIGNFSDV42INTERSSC is enabled, nfsd4putfh() can return success with fhdentry and fhexport both NULL if fhverify() returns nfserrstale and putfh->noverify is true. The NFSD4FHFOREIGN flag is set, but the compound dispatch loop only uses this flag to bypass the nfserrnofilehandle check -- it does not prevent subsequent ops from running with a NULL fhdentry.

A remote client can exploit this by crafting a COMPOUND that includes an inter-SSC COPY (which causes checkifstalefhallowed() to set noverify=true on the saved PUTFH) with an additional op inserted between the source PUTFH and SAVEFH. For example, SETATTR calls fhwantwrite() which dereferences fhexport->expath.mnt without calling fhverify() first, causing a NULL pointer dereference in the nfsd kthread.

Fix this by gating the dispatch loop: when NFSD4FHFOREIGN is set and fhdentry is NULL, only OPSAVEFH (needed for the inter-SSC flow) and ops with ALLOWEDWITHOUTFH (which don't need a resolved filehandle) may proceed. All other ops receive nfserrstale, per RFC 7862 Section 15.2.3 which specifies that foreign filehandle validation is deferred to the consuming operation and NFS4ERRSTALE returned at that point.