[PATCH v4 1/1] scsi: Fix racing between dev init and dev reset

[Alice Chao]

Device reset thread uses kobject_uevent_env() to get kobj.parent, and it
races with device init thread which calls device_add() to add kobj.parent
before kobject_uevent_env().

Device init call:           Device reset call:
 scsi_probe_and_add_lun()    scsi_evt_thread()
  scsi_add_lun()             scsi_evt_emit()
   scsi_sysfs_add_sdev()      kobject_uevent_env() //get kobj.parent
    scsi_target_add()           kobject_get_path()
                                 len = get_kobj_path_length () // len=1 because parent hasn't created yet
    device_add() // add kobj.parent
      kobject_uevent_env()
       kobject_get_path()         path = kzalloc()
        fill_kobj_path()           fill_kobj_path() // --length; length -= cur is a negative value
                                    memcpy(path + length, kobject_name(parent), cur); // slab OOB!

Above backtrace describes the problem, device reset thread will get wrong
kobj.parent when device init thread didn’t add kobj.parent yet. When this
racing happened, it triggers the a KASAN dump on the final iteration:

BUG: KASAN: slab-out-of-bounds in kobject_get_path+0xf8/0x1b8
Write of size 11 at addr ffffff80d6bb94f5 by task kworker/3:1/58
<snip>
Call trace:
 __kasan_report+0x124/0x1c8
 kasan_report+0x54/0x84
 kasan_check_range+0x200/0x208
 memcpy+0xb8/0xf0
 kobject_get_path+0xf8/0x1b8
 kobject_uevent_env+0x228/0xa88
 scsi_evt_thread+0x2d0/0x5b0
 process_one_work+0x570/0xf94
 worker_thread+0x7cc/0xf80
 kthread+0x2c4/0x388

These two jobs are scheduled asynchronously, we can't guaranteed that
kobj.parent will be created in device init thread before device reset
thread calls kobject_get_path().

To resolve the racing issue between device init thread and device reset
thread, we use wait_event() in scsi_evt_emit() to wait for device_add()
to complete the creation of kobj.parent.

Device init call:                Device reset call:
ufshcd_async_scan()              scsi_evt_thread()
 scsi_scan_host()                 scsi_evt_emit() <- add wait_event()
  do_scsi_scan_host() <- add wake_up()
   scsi_scan_host_selected()
    scsi_scan_channel()
     scsi_probe_and_add_lun()
      scsi_target_add()
       device_add() // add kobj.parent
        kobject_uevent_env()
         kobject_get_path()
          fill_kobj_path()
  // call wake_up() after scsi_scan_host_selected is done
                                   kobject_uevent_env() // add kobj.parent
                                    kobject_get_path() // get valid kobj.parent
                                     fill_kobj_path()

After we add wake_up at do_scsi_scan_host() in device init thread, we can
ensure that device reset thread will get kobject after device init thread
finishes adding parent.

Signed-off-by: Alice Chao <alice.chao at mediatek.com>

---

Change in v4
-Change commit: Change call stack description.

Change in v3
-Change commit: Describe the preblem first and then the solution.
-Add commit: Add KASAN error log.

Change in v2
-Remove Change-Id.

---
 drivers/scsi/scsi_lib.c  | 1 +
 drivers/scsi/scsi_scan.c | 1 +
 2 files changed, 2 insertions(+)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 0a70aa763a96..abf9a71ed77c 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -2461,6 +2461,7 @@ static void scsi_evt_emit(struct scsi_device *sdev, struct scsi_event *evt)
 		break;
 	case SDEV_EVT_POWER_ON_RESET_OCCURRED:
 		envp[idx++] = "SDEV_UA=POWER_ON_RESET_OCCURRED";
+		wait_event(sdev->host->host_wait, sdev->sdev_gendev.kobj.parent != NULL);
 		break;
 	default:
 		/* do nothing */
diff --git a/drivers/scsi/scsi_scan.c b/drivers/scsi/scsi_scan.c
index f4e6c68ac99e..431f229ac435 100644
--- a/drivers/scsi/scsi_scan.c
+++ b/drivers/scsi/scsi_scan.c
@@ -1904,6 +1904,7 @@ static void do_scsi_scan_host(struct Scsi_Host *shost)
 	} else {
 		scsi_scan_host_selected(shost, SCAN_WILD_CARD, SCAN_WILD_CARD,
 				SCAN_WILD_CARD, 0);
+		wake_up(&shost->host_wait);
 	}
 }
 
-- 
2.18.0