CVE-2026-89696

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

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

When CONFIG NFSD V4 2 INTER SSC is enabled, nfsd4 putfh() can return success with fh dentry and fh export both NULL if fh verify() returns nfserr stale and putfh->no verify is true. The NFSD4 FH FOREIGN flag is set, but the compound dispatch loop only uses this flag to bypass the nfserr nofilehandle check -- it does not prevent subsequent ops from running with a NULL fh dentry.

A remote client can exploit this by crafting a COMPOUND that includes an inter-SSC COPY (which causes check if stalefh allowed() to set no verify=true on the saved PUTFH) with an additional op inserted between the source PUTFH and SAVEFH. For example, SETATTR calls fh want write() which dereferences fh export->ex path.mnt without calling fh verify() first, causing a NULL pointer dereference in the nfsd kthread.

Fix this by gating the dispatch loop: when NFSD4 FH FOREIGN is set and fh dentry is NULL, only OP SAVEFH (needed for the inter-SSC flow) and ops with ALLOWED WITHOUT FH (which don't need a resolved filehandle) may proceed. All other ops receive nfserr stale, per RFC 7862 Section 15.2.3 which specifies that foreign filehandle validation is deferred to the consuming operation and NFS4ERR STALE returned at that point.