usb: gadget: f_tcm: keep port count until LUN teardown completes

commit c39d091 upstream.

tcm_usbg_drop_nexus() permits session removal once tpg_port_count
reaches zero. However, usbg_port_unlink() currently decrements that
count from the fabric_pre_unlink() callback, before core_dev_del_lun()
waits for active se_lun references to drain.

If removal of the last LUN races a nexus removal, the latter can observe
a zero port count and call target_remove_session(). This frees
sess_cmd_map while an in-flight struct usbg_cmd, including its work item,
can still be accessed.

Overlapping the last-LUN unlink with nexus removal reproduces this
lifetime violation as a DEBUG_OBJECTS "free active" warning for
usbg_cmd_work, followed by a target-core BUG/Oops.

The generic target-core unlink path has no callback after
core_dev_del_lun() completes. Add an optional fabric_post_unlink()
callback and use it for the f_tcm port count. The count now remains
nonzero until core_dev_del_lun() has finished draining active LUN
references, preventing nexus removal from freeing the session during
command completion.

Fixes: c52661d ("usb-gadget: Initial merge of target module for UASP + BOT")
Cc: stable@vger.kernel.org
Signed-off-by: Shuangpeng Bai <shuangpeng.kernel@gmail.com>
Link: https://patch.msgid.link/20260807060733.3186624-1-shuangpeng.kernel@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: shuangpeng-kernel

drivers/target/target_core_fabric_configfs.c

@@ -690,6 +690,14 @@ static void target_fabric_port_unlink(
 	}
 
 	core_dev_del_lun(se_tpg, lun);
+
+	if (tf->tf_ops->fabric_post_unlink) {
+		/*
+		 * Allow fabrics to release state that must remain valid until
+		 * core_dev_del_lun() has drained all active LUN references.
+		 */
+		tf->tf_ops->fabric_post_unlink(se_tpg, lun);
+	}
 }
 
 static void target_fabric_port_release(struct config_item *item)

drivers/usb/gadget/function/f_tcm.c

@@ -2023,7 +2023,7 @@ static const struct target_core_fabric_ops usbg_ops = {
 	.fabric_enable_tpg		= usbg_enable_tpg,
 	.fabric_drop_tpg		= usbg_drop_tpg,
 	.fabric_post_link		= usbg_port_link,
-	.fabric_pre_unlink		= usbg_port_unlink,
+	.fabric_post_unlink		= usbg_port_unlink,
 	.fabric_init_nodeacl		= usbg_init_nodeacl,
 
 	.tfc_wwn_attrs			= usbg_wwn_attrs,

include/target/target_core_fabric.h

@@ -95,6 +95,8 @@ struct target_core_fabric_ops {
 				struct se_lun *);
 	void (*fabric_pre_unlink)(struct se_portal_group *,
 				struct se_lun *);
+	void (*fabric_post_unlink)(struct se_portal_group *se_tpg,
+				   struct se_lun *lun);
 	struct se_tpg_np *(*fabric_make_np)(struct se_portal_group *,
 				struct config_group *, const char *);
 	void (*fabric_drop_np)(struct se_tpg_np *);