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.

Fixes: 7e13f4f8d2 ("nfsd: handle delegated timestamps in SETATTR")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-sonnet-4-6
Signed-off-by: Nikol Kuklev <nikolk202@gmail.com>
Signed-off-by: Chuck Lever <cel@kernel.org>

fs/nfsd/nfs4proc.c

@@ -1262,7 +1262,7 @@ nfsd4_setattr(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 
 	if (deleg_attrs) {
 		status = nfserr_bad_stateid;
-		if (st->sc_type & SC_TYPE_DELEG) {
+		if (st && (st->sc_type & SC_TYPE_DELEG)) {
 			struct nfs4_delegation *dp = delegstateid(st);
 
 			/* Only for *_ATTRS_DELEG flavors */