[PATCH v3 04/10] nfsd: initialize copy-notify stateid before publishing it

[Jeff Layton <jlayton@kernel.org>]

nfsd4_copy_notify() finished initializing the cpntf state after
nfs4_alloc_init_cpntf_state() had already linked it into the
s2s_cp_stateids IDR and the parent's sc_cp_list, with cs_count == 1 (the
membership reference) and none held for the caller. A racing
OFFLOAD_CANCEL (crafted cl_id == nn->s2s_cp_cl_id plus the guessable
so_id) could reach manage_cpntf_state() and free the entry, turning the
caller's subsequent cpn_cnr_stateid read and cp_p_stateid/cp_p_clid
writes into use-after-free. The owning clientid was also only recorded
after publication, so it could not gate an ownership check in that window.

Record cp_p_stateid and cp_p_clid inside nfs4_alloc_init_cpntf_state()
before nfs4_init_cp_state() publishes the entry, and return it with an
extra reference. The caller reads the stateid under that reference and
drops it with nfs4_put_cpntf_state(); on a late error the laundromat
reaps the entry.

Fixes: 624322f1adc5 ("NFSD add COPY_NOTIFY operation")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/nfsd/nfs4proc.c  | 16 +++++++++-------
 fs/nfsd/nfs4state.c | 10 +++++++++-
 2 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
index b9049a3d7c07..506cd4910fb7 100644
--- a/fs/nfsd/nfs4proc.c
+++ b/fs/nfsd/nfs4proc.c
@@ -2373,7 +2373,6 @@ nfsd4_copy_notify(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	struct nfsd_net *nn = net_generic(SVC_NET(rqstp), nfsd_net_id);
 	struct nfs4_stid *stid = NULL;
 	struct nfs4_cpntf_state *cps;
-	struct nfs4_client *clp = cstate->clp;
 
 	status = nfs4_preprocess_stateid_op(rqstp, cstate, &cstate->current_fh,
 					&cn->cpn_src_stateid, RD_STATE, NULL,
@@ -2387,12 +2386,14 @@ nfsd4_copy_notify(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	cn->cpn_lease_time.tv_nsec = 0;
 
 	status = nfserrno(-ENOMEM);
+	/*
+	 * The returned cps is published and fully initialized, and carries an
+	 * extra reference for us; drop it once we are done with it.
+	 */
 	cps = nfs4_alloc_init_cpntf_state(nn, stid);
 	if (!cps)
 		goto out;
 	memcpy(&cn->cpn_cnr_stateid, &cps->cp_stateid.cs_stid, sizeof(stateid_t));
-	memcpy(&cps->cp_p_stateid, &stid->sc_stateid, sizeof(stateid_t));
-	memcpy(&cps->cp_p_clid, &clp->cl_clientid, sizeof(clientid_t));
 
 	/* For now, only return one server address in cpn_src, the
 	 * address used by the client to connect to this server.
@@ -2401,10 +2402,11 @@ nfsd4_copy_notify(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	status = nfsd4_set_netaddr((struct sockaddr *)&rqstp->rq_daddr,
 				 &cn->cpn_src->u.nl4_addr);
 	WARN_ON_ONCE(status);
-	if (status) {
-		nfs4_put_cpntf_state(nn, cps);
-		goto out;
-	}
+	/*
+	 * Drop our extra reference. The membership reference keeps the entry
+	 * alive for a later inter-server READ, or until the laundromat reaps it.
+	 */
+	nfs4_put_cpntf_state(nn, cps);
 out:
 	nfs4_put_stid(stid);
 	return status;
diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 3f83c5107e28..9ae0f18121ef 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -995,7 +995,15 @@ struct nfs4_cpntf_state *nfs4_alloc_init_cpntf_state(struct nfsd_net *nn,
 	/* So a stale list_del_init() before linking is a no-op. */
 	INIT_LIST_HEAD(&cps->cp_list);
 	cps->cpntf_time = ktime_get_boottime_seconds();
-	refcount_set(&cps->cp_stateid.cs_count, 1);
+	/*
+	 * Fully initialize the entry before nfs4_init_cp_state() publishes it,
+	 * since a concurrent OFFLOAD_CANCEL could then free it. Take an extra
+	 * reference for the caller (dropped with nfs4_put_cpntf_state()).
+	 */
+	memcpy(&cps->cp_p_stateid, &p_stid->sc_stateid, sizeof(stateid_t));
+	memcpy(&cps->cp_p_clid, &p_stid->sc_client->cl_clientid,
+	       sizeof(clientid_t));
+	refcount_set(&cps->cp_stateid.cs_count, 2);
 	if (!nfs4_init_cp_state(nn, &cps->cp_stateid, NFS4_COPYNOTIFY_STID,
 				p_stid))
 		goto out_free;

-- 
2.55.0