Date:   Mon, 9 Mar 2020 13:35:22 -0700
From:   Brendan Higgins <brendanhiggins@...gle.com>
To:     Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc:     Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        hdegoede@...hat.com,
        "rafael.j.wysocki" <rafael.j.wysocki@...el.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Shuah Khan <shuah@...nel.org>,
        Anders Roxell <anders.roxell@...aro.org>,
        lkft-triage@...ts.linaro.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: BUG: kernel NULL pointer dereference, address: 00 - ida_free+0x76/0x140

On Fri, Mar 6, 2020 at 4:05 AM Heikki Krogerus
<heikki.krogerus@...ux.intel.com> wrote:
>
> On Fri, Mar 06, 2020 at 12:33:50AM +0200, Sakari Ailus wrote:
> > Hi Brendan,
> >
> > On Thu, Mar 05, 2020 at 11:51:20AM -0800, Brendan Higgins wrote:
> > > On Thu, Mar 5, 2020 at 11:40 AM Brendan Higgins
> > > <brendanhiggins@...gle.com> wrote:
> > > >
> > > > On Thu, Mar 5, 2020 at 11:18 AM Andy Shevchenko
> > > > <andy.shevchenko@...il.com> wrote:
> > > > >
> > > > > +Cc: Sakari
> > > > >
> > > > > On Thu, Mar 5, 2020 at 6:00 PM Naresh Kamboju <naresh.kamboju@...aro.org> wrote:
> > > > > >
> > > > > > Regression reported on Linux next 5.6.0-rc4-next-20200305 on x86_64,
> > > > > > i386, arm and arm64. The steps to reproduce is running kselftests lib
> > > > > > printf.sh test case.
> > > > > > Which is doing modprobe operations.
> > > > > >
> > > > > > BTW, there are few RCU warnings from the boot log.
> > > > > > Please refer below link for more details.
> > > > > >
> > > > > > Steps reproduce by using kselftests,
> > > > > >
> > > > > >           - lsmod || true
> > > > > >           - cd /opt/kselftests/default-in-kernel/lib/
> > > > > >           - export PATH=/opt/kselftests/default-in-kernel/kselftest:$PATH
> > > > > >           - ./printf.sh || true
> > > > > >           - ./bitmap.sh || true
> > > > > >           - ./prime_numbers.sh || true
> > > > > >           - ./strscpy.sh || true
> > > > > >
> > > > > > x86_64 kernel BUG dump.
> > > > > > + ./printf.sh
> > > >
> > > > Oops, I am wondering if I broke this with my change "Revert "software
> > > > node: Simplify software_node_release() function"":
> > > >
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=d1c19322388d6935b534b494a2c223dd089e30dd
> > > >
> > > > I am still investigating, will update later.
> > >
> > > Okay, yeah, I am pretty sure I caused the breakage. I got an email
> > > from kernel test robot a couple days ago that I didn't see:
> > >
> > > https://lists.01.org/hyperkitty/list/lkp@lists.01.org/thread/N3ZN5XH7HK24JVEJ5WSQD2SK6YCDRILR/
> > >
> > > It shows the same breakage after applying this change.
> > >
> > > I am still investigating how my change broke it, nevertheless.
> >
> > As nodes in the tree are being removed, the code before the patch that
> > "simplified" the software_node_release() function accessed the node's parent
> > in its release function.
> >
> > And if CONFIG_DEBUG_KOBJECT_RELEASE is defined, the release functions are no
> > longer necessarily called in order, leading to referencing released memory.
> > Oops!
> >
> > So Heikki's patch actually fixed a bug. :-)
>
> Well, I think it just hid the problem. It looks like the core
> (lib/kobject.c) allows the parent kobject to be released before the
> last child kobject is released. To be honest, that does not sound
> right to me...
>
> I think we can workaround this problem by taking reference to the
> parent when the child is added, and then releasing it when the child
> is released, and in that way be guaranteed that the parent will not
> disappear before the child is fully released, but that still does not
> feel right. It feels more like the core is not doing it's job to me.
> The parent just should not be released before its children.
>
> Either I'm wrong about that, and we still should take the reference on
> the parent, or we revert my patch like Brendan proposed and then fix

Either way, isn't it wrong to release the node ID before deleting the
sysfs entry? I am not sure that my fix was the correct one, but I
believe the bug that Heidi and I found is actually a bug.

> the core with something like this (warning, I did not even try to
> compile that):

I will try it out.

> diff --git a/lib/kobject.c b/lib/kobject.c
> index 83198cb37d8d..ec5774992337 100644
> --- a/lib/kobject.c
> +++ b/lib/kobject.c
> @@ -680,6 +680,12 @@ static void kobject_cleanup(struct kobject *kobj)
>                 kobject_uevent(kobj, KOBJ_REMOVE);
>         }
>
> +       if (t && t->release) {
> +               pr_debug("kobject: '%s' (%p): calling ktype release\n",
> +                        kobject_name(kobj), kobj);
> +               t->release(kobj);
> +       }
> +
>         /* remove from sysfs if the caller did not do it */
>         if (kobj->state_in_sysfs) {
>                 pr_debug("kobject: '%s' (%p): auto cleanup kobject_del\n",
> @@ -687,12 +693,6 @@ static void kobject_cleanup(struct kobject *kobj)
>                 kobject_del(kobj);
>         }
>
> -       if (t && t->release) {
> -               pr_debug("kobject: '%s' (%p): calling ktype release\n",
> -                        kobject_name(kobj), kobj);
> -               t->release(kobj);
> -       }
> -
>         /* free name if we allocated it */
>         if (name) {
>                 pr_debug("kobject: '%s': free name\n", name);
>

Thanks!

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