linux-kernel - Re: [PATCH v4 1/6] timer: kasan: record timer stack

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Date:   Fri, 25 Sep 2020 15:18:43 +0800
From:   Walter Wu <walter-zh.wu@...iatek.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Marco Elver <elver@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        "Alexander Potapenko" <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "Andrey Konovalov" <andreyknvl@...gle.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        <kasan-dev@...glegroups.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        wsd_upstream <wsd_upstream@...iatek.com>,
        <linux-mediatek@...ts.infradead.org>
Subject: Re: [PATCH v4 1/6] timer: kasan: record timer stack

On Thu, 2020-09-24 at 23:41 +0200, Thomas Gleixner wrote:
> On Thu, Sep 24 2020 at 12:03, Walter Wu wrote:
> > When analyze use-after-free or double-free issue, recording the timer
> > stacks is helpful to preserve usage history which potentially gives
> > a hint about the affected code.
> >
> > Record the most recent two timer init calls in KASAN which are printed
> > on failure in the KASAN report.
> >
> > For timers it has turned out to be useful to record the stack trace
> > of the timer init call.
> 
> In which way? And what kind of bug does it catch which cannot be catched
> by existing debug mechanisms already?
> 

We only provide another debug mechanisms to debug use-after-free or
double-free, it can be displayed together in KASAN report and have a
chance to debug, and it doesn't need to enable existing debug mechanisms
at the same time. then it has a chance to resolve issue.

> > Because if the UAF root cause is in timer init, then user can see
> > KASAN report to get where it is registered and find out the root
> > cause.
> 
> What? If the UAF root cause is in timer init, then registering it after
> using it in that very same function is pretty pointless.
> 

See [1], the call stack shows UAF happen at dummy_timer(), it is the
callback function and set by timer_setup(), if KASAN report shows the
timer call stack, it should be useful for programmer.

[1]
https://syzkaller.appspot.com/bug?id=34e69b7c8c0165658cbc987da0b61dadec644b6b


> > It don't need to enable DEBUG_OBJECTS_TIMERS, but they have a chance
> > to find out the root cause.
> 
> There is a lot of handwaving how useful this is, but TBH I don't see the
> value at all.
> 
> DEBUG_OBJECTS_TIMERS does a lot more than crashing on UAF. If KASAN
> provides additional value over DEBUG_OBJECTS_TIMERS then spell it out,
> but just saying that you don't need to enable DEBUG_OBJECTS_TIMERS is
> not making an argument for that change.
> 

We don't want to replace DEBUG_OBJECTS_TIMERS with this patches, only
hope to use low overhead(compare with DEBUG_OBJECTS_TIMERS) to debug
use-after-free/double-free issue. If you have some concerns, we can add
those message into commit log.

Thanks.

Walter