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Date:   Thu, 28 Feb 2019 19:01:49 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Julien Thierry <julien.thierry@....com>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>, valentin.schneider@....com,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/8] kasan,x86: Frob kasan_report() in an exception



On 2/28/19 6:52 PM, Dmitry Vyukov wrote:
> On Thu, Feb 28, 2019 at 4:46 PM Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> On Thu, Feb 28, 2019 at 04:22:04PM +0100, Dmitry Vyukov wrote:
>>> On Thu, Feb 28, 2019 at 4:05 PM Peter Zijlstra <peterz@...radead.org> wrote:
>>>>
>>>> Because __asan_{load,store}{N,1,2,4,8,16}_noabort() get called from
>>>> UACCESS context, and kasan_report() is most definitely _NOT_ safe to
>>>> be called from there, move it into an exception much like BUG/WARN.
>>>>
>>>> *compile tested only*
>>>
>>>
>>> Please test it by booting KASAN kernel and then loading module
>>> produced by CONFIG_TEST_KASAN=y. There are too many subtle aspects to
>>> rely on "compile tested only", reviewers can't catch all of them
>>> either.
>>
>> Sure, I'll do that. I just wanted to share the rest of the patches.
>>
>> A quick test shows it dies _REAAAAAAAALY_ early, as in:
>>
>> "Booting the kernel."
>>
>> is the first and very last thing it says... I wonder how I did that :-)
> 
> One thing is that during early boot kasan_report is called multiple
> times, but these are false positives related to the fact that we don't
> have a proper shadow yet (setup later). So during early boot we set
> kasan_disable=1 (or some global or per-task flag), and then
> kasan_report checks it and returns.
> Once we setup proper shadow, the flag is reset and from now on
> kasan_report actually reports bug.
> 

Yup, see report_enabled() function.

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