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

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Date:   Thu, 24 Sep 2020 23:41:27 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Walter Wu <walter-zh.wu@...iatek.com>,
        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>
Cc:     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,
        Walter Wu <walter-zh.wu@...iatek.com>
Subject: Re: [PATCH v4 1/6] timer: kasan: record timer stack

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?

> 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.

> 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.

Try again please.

Thanks,

        tglx