[PATCH-v2 4/4] target: Obtain se_node_acl->acl_kref during get_initiator_node_acl

[Nicholas A. Bellinger]

From: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>

This patch addresses a long standing race where obtaining
se_node_acl->acl_kref in __transport_register_session()
happens a bit too late, and leaves open the potential
for core_tpg_del_initiator_node_acl() to hit a NULL
pointer dereference.

Instead, take ->acl_kref in core_tpg_get_initiator_node_acl()
while se_portal_group->acl_node_mutex is held, and move the
final target_put_nacl() from transport_deregister_session()
into transport_free_session() so that fabric driver login
failure handling using the modern method to still work
as expected.

Also, update core_tpg_check_initiator_node_acl() to take
an extra reference for dynamically generated acls for
demo-mode, before returning to fabric caller.

Note the existing wait_for_completion(&acl->acl_free_comp)
in core_tpg_del_initiator_node_acl() does not change.

Cc: Sagi Grimberg <sagig@xxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxx>
Cc: Andy Grover <agrover@xxxxxxxxxx>
Signed-off-by: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>
---
 drivers/target/target_core_tpg.c       | 18 +++++++++++++++++-
 drivers/target/target_core_transport.c | 18 ++++++++++++------
 2 files changed, 29 insertions(+), 7 deletions(-)

diff --git a/drivers/target/target_core_tpg.c b/drivers/target/target_core_tpg.c
index 66a2c6f..5221aee 100644
--- a/drivers/target/target_core_tpg.c
+++ b/drivers/target/target_core_tpg.c
@@ -75,9 +75,16 @@ struct se_node_acl *core_tpg_get_initiator_node_acl(
 	unsigned char *initiatorname)
 {
 	struct se_node_acl *acl;
-
+	/*
+	 * Obtain the acl_kref now, which will be dropped upon the
+	 * release of se_sess memory within transport_free_session().
+	 */
 	mutex_lock(&tpg->acl_node_mutex);
 	acl = __core_tpg_get_initiator_node_acl(tpg, initiatorname);
+	if (acl) {
+		if (!kref_get_unless_zero(&acl->acl_kref))
+			acl = NULL;
+	}
 	mutex_unlock(&tpg->acl_node_mutex);
 
 	return acl;
@@ -240,6 +247,15 @@ struct se_node_acl *core_tpg_check_initiator_node_acl(
 	acl = target_alloc_node_acl(tpg, initiatorname);
 	if (!acl)
 		return NULL;
+	/*
+	 * When allocating a dynamically generated node_acl, go ahead
+	 * and take the extra kref now before returning to the fabric
+	 * driver caller.
+	 *
+	 * Note this reference will be released at session shutdown
+	 * time within transport_free_session() code.
+	 */
+	kref_get(&acl->acl_kref);
 	acl->dynamic_node_acl = 1;
 
 	/*
diff --git a/drivers/target/target_core_transport.c b/drivers/target/target_core_transport.c
index 7b05ebf..c5035b9 100644
--- a/drivers/target/target_core_transport.c
+++ b/drivers/target/target_core_transport.c
@@ -341,7 +341,6 @@ void __transport_register_session(
 					&buf[0], PR_REG_ISID_LEN);
 			se_sess->sess_bin_isid = get_unaligned_be64(&buf[0]);
 		}
-		kref_get(&se_nacl->acl_kref);
 
 		spin_lock_irq(&se_nacl->nacl_sess_lock);
 		/*
@@ -464,6 +463,15 @@ EXPORT_SYMBOL(transport_deregister_session_configfs);
 
 void transport_free_session(struct se_session *se_sess)
 {
+	struct se_node_acl *se_nacl = se_sess->se_node_acl;
+	/*
+	 * Drop the se_node_acl->nacl_kref obtained from within
+	 * core_tpg_get_initiator_node_acl().
+	 */
+	if (se_nacl) {
+		se_sess->se_node_acl = NULL;
+		target_put_nacl(se_nacl);
+	}
 	if (se_sess->sess_cmd_map) {
 		percpu_ida_destroy(&se_sess->sess_tag_pool);
 		kvfree(se_sess->sess_cmd_map);
@@ -478,7 +486,7 @@ void transport_deregister_session(struct se_session *se_sess)
 	const struct target_core_fabric_ops *se_tfo;
 	struct se_node_acl *se_nacl;
 	unsigned long flags;
-	bool comp_nacl = true, drop_nacl = false;
+	bool drop_nacl = false;
 
 	if (!se_tpg) {
 		transport_free_session(se_sess);
@@ -510,18 +518,16 @@ void transport_deregister_session(struct se_session *se_sess)
 	if (drop_nacl) {
 		core_tpg_wait_for_nacl_pr_ref(se_nacl);
 		core_free_device_list_for_node(se_nacl, se_tpg);
+		se_sess->se_node_acl = NULL;
 		kfree(se_nacl);
-		comp_nacl = false;
 	}
 	pr_debug("TARGET_CORE[%s]: Deregistered fabric_sess\n",
 		se_tpg->se_tpg_tfo->get_fabric_name());
 	/*
 	 * If last kref is dropping now for an explicit NodeACL, awake sleeping
 	 * ->acl_free_comp caller to wakeup configfs se_node_acl->acl_group
-	 * removal context.
+	 * removal context from within transport_free_session() code.
 	 */
-	if (se_nacl && comp_nacl)
-		target_put_nacl(se_nacl);
 
 	transport_free_session(se_sess);
 }
-- 
1.9.1