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

Overview

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.

Affected

• Linux >= b9e8638e3d9ed8334f1f7071e081860aac37e83e < ffdc844e673d2bcb2af8c8c3eef8cdea59d0bf32
• Linux >= b9e8638e3d9ed8334f1f7071e081860aac37e83e < 00c84f4eec715e501efc080a5670114189e42507
• Linux >= b9e8638e3d9ed8334f1f7071e081860aac37e83e < 2d9846fd1c767920fddd4bde30eb35cd2969df13
• Linux >= b9e8638e3d9ed8334f1f7071e081860aac37e83e < 977e6f006a7a3ffc4216ae6034f768f8de6fd138
• Linux >= b9e8638e3d9ed8334f1f7071e081860aac37e83e < 35f248bd40b47b229d4999581df45b97daadd977
• Linux >= b9e8638e3d9ed8334f1f7071e081860aac37e83e < 311f7d926630940650447cbd1c932b076b40a6c4
• Linux >= b9e8638e3d9ed8334f1f7071e081860aac37e83e < bf4d338dc8625d70c7f2cb0657d66851a7ac9154
• Linux >= b9e8638e3d9ed8334f1f7071e081860aac37e83e < c59738a00aa51b16adc1b5ceb7c80877168efb4d
• Linux 5.6

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Vendor advisories

• Red Hat VEX · Moderate · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4 · no fix planned: Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4 · updated 2026-09-21 · vex