nfsd: fix null dereference in nfsd4_setattr for deleg timestamp attrs

Overview

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

nfsd: fix null dereference in nfsd4_setattr for deleg timestamp attrs

When a SETATTR request includes FATTR4_WORD2_TIME_DELEG_ACCESS or FATTR4_WORD2_TIME_DELEG_MODIFY in the attribute bitmap, nfsd4_setattr() sets deleg_attrs=true and calls nfs4_preprocess_stateid_op() to validate the stateid.

If the client supplies the NFSv4 "one stateid" (all-0xFF bytes), check_special_stateids() returns nfs_ok without populating the output nfs4_stid pointer, because the special-stateid path in nfs4_preprocess_stateid_op() jumps to done: with s==NULL, and the "if (s)" block that would set *cstid is skipped. The local variable st remains NULL.

Back in nfsd4_setattr(), the if (deleg_attrs) block then unconditionally dereferences st->sc_type (at offset 4 from NULL), causing a kernel oops.

This is remotely triggerable by any NFSv4 client: send COMPOUND [PUTROOTFH, SETATTR(ONE_STATEID, {bmval2=FATTR4_WORD2_TIME_DELEG_ACCESS, ...})]. No authentication, delegation, or prior state is required.

Fix by adding a NULL check before the dereference. A special stateid is not a delegation stateid, so the existing nfserr_bad_stateid return value is already correct; we only need to guard the pointer dereference itself.

Affected

• Linux >= 7e13f4f8d27dc02fb88666f603c53ca749d56f92 < 3c5119b799a7f57db332b2c31e806de16c869f0f
• Linux >= 7e13f4f8d27dc02fb88666f603c53ca749d56f92 < 3c896db123892256fd39af0ee8c2d02c63539983
• Linux >= 7e13f4f8d27dc02fb88666f603c53ca749d56f92 < fe456c8c0931bb3e8a03d429920e87fd85747fba
• Linux 6.14

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 · no fix planned: Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 10 · updated 2026-09-21 · vex