[PATCH 1/1] scsi: Add length check to prevent invalid memory access

[Alice Chao <alice.chao@mediatek.com>]

Device reset thread uses kobject_uevent_env() to get kobj.parent(kobj.p)
, 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 prevent length -= cur from being a negative value, we add length
check in fill_kobj_path() to prevent invalid memory access.

Signed-off-by: Alice Chao <alice.chao@mediatek.com>
---
 lib/kobject.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/lib/kobject.c b/lib/kobject.c
index af1f5f2954d4..3cccb8e88d4e 100644
--- a/lib/kobject.c
+++ b/lib/kobject.c
@@ -121,6 +121,10 @@ static void fill_kobj_path(struct kobject *kobj, char *path, int length)
 		int cur = strlen(kobject_name(parent));
 		/* back up enough to print this name with '/' */
 		length -= cur;
+
+		if (length <= 0)
+			break;
+
 		memcpy(path + length, kobject_name(parent), cur);
 		*(path + --length) = '/';
 	}
-- 
2.18.0