scsi: target/core: Make sure that target_wait_for_sess_cmds() waits long enough

A session must only be released after all code that accesses the session
structure has finished. Make sure that this is the case by introducing a
new command counter per session that is only decremented after the
.release_cmd() callback has finished. This patch fixes the following crash:

BUG: KASAN: use-after-free in do_raw_spin_lock+0x1c/0x130
Read of size 4 at addr ffff8801534b16e4 by task rmdir/14805
CPU: 16 PID: 14805 Comm: rmdir Not tainted 4.18.0-rc2-dbg+ #5
Call Trace:
dump_stack+0xa4/0xf5
print_address_description+0x6f/0x270
kasan_report+0x241/0x360
__asan_load4+0x78/0x80
do_raw_spin_lock+0x1c/0x130
_raw_spin_lock_irqsave+0x52/0x60
srpt_set_ch_state+0x27/0x70 [ib_srpt]
srpt_disconnect_ch+0x1b/0xc0 [ib_srpt]
srpt_close_session+0xa8/0x260 [ib_srpt]
target_shutdown_sessions+0x170/0x180 [target_core_mod]
core_tpg_del_initiator_node_acl+0xf3/0x200 [target_core_mod]
target_fabric_nacl_base_release+0x25/0x30 [target_core_mod]
config_item_release+0x9c/0x110 [configfs]
config_item_put+0x26/0x30 [configfs]
configfs_rmdir+0x3b8/0x510 [configfs]
vfs_rmdir+0xb3/0x1e0
do_rmdir+0x262/0x2c0
do_syscall_64+0x77/0x230
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Cc: Nicholas Bellinger <[e-mail]>
Cc: Mike Christie <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: David Disseldorp <[e-mail]>
Cc: Hannes Reinecke <[e-mail]>
Signed-off-by: Bart Van Assche <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/target/target_core_transport.c

···
 	sub_api_initialized = 1;
 }
 
+static void target_release_sess_cmd_refcnt(struct percpu_ref *ref)
+{
+	struct se_session *sess = container_of(ref, typeof(*sess), cmd_count);
+
+	wake_up(&sess->cmd_list_wq);
+}
+
 /**
  * transport_init_session - initialize a session object
  * @se_sess: Session object pointer.
  *
  * The caller must have zero-initialized @se_sess before calling this function.
  */
-void transport_init_session(struct se_session *se_sess)
+int transport_init_session(struct se_session *se_sess)
 {
 	INIT_LIST_HEAD(&se_sess->sess_list);
 	INIT_LIST_HEAD(&se_sess->sess_acl_list);
 	INIT_LIST_HEAD(&se_sess->sess_cmd_list);
 	spin_lock_init(&se_sess->sess_cmd_lock);
 	init_waitqueue_head(&se_sess->cmd_list_wq);
+	return percpu_ref_init(&se_sess->cmd_count,
+			       target_release_sess_cmd_refcnt, 0, GFP_KERNEL);
 }
 EXPORT_SYMBOL(transport_init_session);
 
···
 struct se_session *transport_alloc_session(enum target_prot_op sup_prot_ops)
 {
 	struct se_session *se_sess;
+	int ret;
 
 	se_sess = kmem_cache_zalloc(se_sess_cache, GFP_KERNEL);
 	if (!se_sess) {
···
 				" se_sess_cache\n");
 		return ERR_PTR(-ENOMEM);
 	}
-	transport_init_session(se_sess);
+	ret = transport_init_session(se_sess);
+	if (ret < 0) {
+		kfree(se_sess);
+		return ERR_PTR(ret);
+	}
 	se_sess->sup_prot_ops = sup_prot_ops;
 
 	return se_sess;
···
 		sbitmap_queue_free(&se_sess->sess_tag_pool);
 		kvfree(se_sess->sess_cmd_map);
 	}
+	percpu_ref_exit(&se_sess->cmd_count);
 	kmem_cache_free(se_sess_cache, se_sess);
 }
 EXPORT_SYMBOL(transport_free_session);
···
 	}
 	se_cmd->transport_state |= CMD_T_PRE_EXECUTE;
 	list_add_tail(&se_cmd->se_cmd_list, &se_sess->sess_cmd_list);
+	percpu_ref_get(&se_sess->cmd_count);
 out:
 	spin_unlock_irqrestore(&se_sess->sess_cmd_lock, flags);
 
···
 	if (se_sess) {
 		spin_lock_irqsave(&se_sess->sess_cmd_lock, flags);
 		list_del_init(&se_cmd->se_cmd_list);
-		if (se_sess->sess_tearing_down && list_empty(&se_sess->sess_cmd_list))
-			wake_up(&se_sess->cmd_list_wq);
 		spin_unlock_irqrestore(&se_sess->sess_cmd_lock, flags);
 	}
 
···
 	se_cmd->se_tfo->release_cmd(se_cmd);
 	if (compl)
 		complete(compl);
+
+	percpu_ref_put(&se_sess->cmd_count);
 }
 
 /**
···
 	spin_lock_irqsave(&se_sess->sess_cmd_lock, flags);
 	se_sess->sess_tearing_down = 1;
 	spin_unlock_irqrestore(&se_sess->sess_cmd_lock, flags);
+
+	percpu_ref_kill(&se_sess->cmd_count);
 }
 EXPORT_SYMBOL(target_sess_cmd_list_set_waiting);
 
···
 
 	WARN_ON_ONCE(!se_sess->sess_tearing_down);
 
-	spin_lock_irq(&se_sess->sess_cmd_lock);
 	do {
-		ret = wait_event_lock_irq_timeout(
-				se_sess->cmd_list_wq,
-				list_empty(&se_sess->sess_cmd_list),
-				se_sess->sess_cmd_lock, 180 * HZ);
+		ret = wait_event_timeout(se_sess->cmd_list_wq,
+				percpu_ref_is_zero(&se_sess->cmd_count),
+				180 * HZ);
 		list_for_each_entry(cmd, &se_sess->sess_cmd_list, se_cmd_list)
 			target_show_cmd("session shutdown: still waiting for ",
 					cmd);
 	} while (ret <= 0);
-	spin_unlock_irq(&se_sess->sess_cmd_lock);
 }
 EXPORT_SYMBOL(target_wait_for_sess_cmds);
 

drivers/target/target_core_xcopy.c

···
 
 int target_xcopy_setup_pt(void)
 {
+	int ret;
+
 	xcopy_wq = alloc_workqueue("xcopy_wq", WQ_MEM_RECLAIM, 0);
 	if (!xcopy_wq) {
 		pr_err("Unable to allocate xcopy_wq\n");
···
 	INIT_LIST_HEAD(&xcopy_pt_nacl.acl_list);
 	INIT_LIST_HEAD(&xcopy_pt_nacl.acl_sess_list);
 	memset(&xcopy_pt_sess, 0, sizeof(struct se_session));
-	transport_init_session(&xcopy_pt_sess);
+	ret = transport_init_session(&xcopy_pt_sess);
+	if (ret < 0)
+		return ret;
 
 	xcopy_pt_nacl.se_tpg = &xcopy_pt_tpg;
 	xcopy_pt_nacl.nacl_sess = &xcopy_pt_sess;

include/target/target_core_base.h

···
 	struct se_node_acl	*se_node_acl;
 	struct se_portal_group *se_tpg;
 	void			*fabric_sess_ptr;
+	struct percpu_ref	cmd_count;
 	struct list_head	sess_list;
 	struct list_head	sess_acl_list;
 	struct list_head	sess_cmd_list;

include/target/target_core_fabric.h

···
 				struct se_session *, void *));
 void target_remove_session(struct se_session *);
 
-void transport_init_session(struct se_session *);
+int transport_init_session(struct se_session *se_sess);
 struct se_session *transport_alloc_session(enum target_prot_op);
 int transport_alloc_session_tags(struct se_session *, unsigned int,
 		unsigned int);