Re: [PATCH v4] driver core: Fix use-after-free and double free on glue directory

[Muchun Song]

Hi greg k-h,

Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> ä2019å5æ25æåå äå3:04åéï
>
> On Thu, May 16, 2019 at 10:23:42PM +0800, Muchun Song wrote:
> > There is a race condition between removing glue directory and adding a new
> > device under the glue directory. It can be reproduced in following test:
>
> <snip>
>
> Is this related to:
>         Subject: [PATCH v3] drivers: core: Remove glue dirs early only when refcount is 1
>
> ?
>
> If so, why is the solution so different?

In the v1 patch, the solution is that remove glue dirs early only when
refcount is 1. So
the v1 patch like below:

@@ -1825,7 +1825,7 @@ static void cleanup_glue_dir(struct device *dev,
struct kobject *glue_dir)
                return;

        mutex_lock(&gdp_mutex);
-       if (!kobject_has_children(glue_dir))
+       if (!kobject_has_children(glue_dir) && kref_read(&glue_dir->kref) == 1)
                kobject_del(glue_dir);
        kobject_put(glue_dir);
        mutex_unlock(&gdp_mutex);
-----------------------------------------------------------------------

But from Ben's suggestion as below:

I find relying on the object count for such decisions rather fragile as
it could be taken temporarily for other reasons, couldn't it ? In which
case we would just fail...

Ideally, the looking up of the glue dir and creation of its child
should be protected by the same lock instance (the gdp_mutex in that
case).
-----------------------------------------------------------------------

So another solution is used from Ben's suggestion in the v2 patch. But
I forgot to update the commit message until the v4 patch. Thanks.

Yours,
Muchun