[PATCH-next v2] scsi: fix use-after-free problem in scsi_remove_target

[Zhong Jinghua <zhongjinghua@huawei.com>]

A use-after-free problem like below:

BUG: KASAN: use-after-free in scsi_target_reap+0x6c/0x70

Workqueue: scsi_wq_1 __iscsi_unbind_session [scsi_transport_iscsi]
Call trace:
 dump_backtrace+0x0/0x320
 show_stack+0x24/0x30
 dump_stack+0xdc/0x128
 print_address_description+0x68/0x278
 kasan_report+0x1e4/0x308
 __asan_report_load4_noabort+0x30/0x40
 scsi_target_reap+0x6c/0x70
 scsi_remove_target+0x430/0x640
 __iscsi_unbind_session+0x164/0x268 [scsi_transport_iscsi]
 process_one_work+0x67c/0x1350
 worker_thread+0x370/0xf90
 kthread+0x2a4/0x320
 ret_from_fork+0x10/0x18

The problem is caused by a concurrency scenario:

T0: delete target
// echo 1 > /sys/devices/platform/host1/session1/target1:0:0/1:0:0:1/delete
T1: logout
// iscsiadm -m node --logout

T0							T1
 sdev_store_delete
  scsi_remove_device
   device_remove_file
    __scsi_remove_device
        					__iscsi_unbind_session
        					 scsi_remove_target
						  spin_lock_irqsave
        					  list_for_each_entry
     scsi_target_reap
     // starget->reap_ref 1 -> 0
     						  kref_get(&starget->reap_ref);
						  // warn use-after-free.
						  spin_unlock_irqrestore
      scsi_target_reap_ref_release
	scsi_target_destroy
	... // delete starget
						  scsi_target_reap
						  // UAF

When T0 reduces the reference count to 0, but has not been released,
T1 can still enter list_for_each_entry, and then kref_get reports UAF.

Fix it by using kref_get_unless_zero() to check for a reference count of
0.

Signed-off-by: Zhong Jinghua <zhongjinghua@huawei.com>
---
 v2: commit message: "starget->reaf" -> "starget->reap_ref"
 comment: "If it is reduced to 0, it means that other processes are releasing it and there is no need to delete it again"
 ->
 "If the reference count is already zero, skip this target is safe  because scsi_target_destroy() will wait until the 
 host lock has been released before freeing starget."

 drivers/scsi/scsi_sysfs.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c
index e7893835b99a..12e8ed6d55cb 100644
--- a/drivers/scsi/scsi_sysfs.c
+++ b/drivers/scsi/scsi_sysfs.c
@@ -1561,7 +1561,16 @@ void scsi_remove_target(struct device *dev)
 		    starget->state == STARGET_CREATED_REMOVE)
 			continue;
 		if (starget->dev.parent == dev || &starget->dev == dev) {
-			kref_get(&starget->reap_ref);
+
+			/*
+			 * If the reference count is already zero, skip this
+			 * target is safe  because scsi_target_destroy()
+			 * will wait until the host lock has been released
+			 * before freeing starget.
+			 */
+			if (!kref_get_unless_zero(&starget->reap_ref))
+				continue;
+
 			if (starget->state == STARGET_CREATED)
 				starget->state = STARGET_CREATED_REMOVE;
 			else
-- 
2.31.1