linux-kernel - Re: [PATCH v4 00/17] khwasan: kernel hardware assisted address sanitizer

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Message-ID: <CAAeHK+w0T43+h3xqU4a-qutxd-qiEhsvk0eaZpmAn-T0hpaLZQ@mail.gmail.com>
Date:   Thu, 28 Jun 2018 20:56:41 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mark Rutland <mark.rutland@....com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Ingo Molnar <mingo@...nel.org>,
        Paul Lawrence <paullawrence@...gle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
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        Mark Brand <markbrand@...gle.com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Evgeniy Stepanov <eugenis@...gle.com>
Subject: Re: [PATCH v4 00/17] khwasan: kernel hardware assisted address sanitizer

On Thu, Jun 28, 2018 at 12:51 PM, Dave Martin <Dave.Martin@....com> wrote:
> On Tue, Jun 26, 2018 at 03:15:10PM +0200, Andrey Konovalov wrote:
>> 1. By using the Top Byte Ignore arm64 CPU feature, we can store pointer
>>    tags in the top byte of each kernel pointer.
>
> [...]
>
> This is a change from the current situation, so the kernel may be
> making implicit assumptions about the top byte of kernel addresses.
>
> Randomising the top bits may cause things like address conversions and
> pointer arithmetic to break.
>
> For example, (q - p) will not produce the expected result if q and p
> have different tags.

If q and p have different tags, that means they come from different
allocations. I don't think it would make sense to calculate pointer
difference in this case.

>
> Conversions, such as between pointer and pfn, may also go wrong if not
> appropriately masked.
>
> There are also potential pointer comparison and aliasing issues if
> the tag bits are ever stripped or modified.
>
>
> What was your approach to tracking down all the points in the code
> where we have a potential issue?

I've been fuzzing the kernel built with KWHASAN with syzkaller. This
gives a decent coverage and I was able to find some places where
fixups were required this way. Right now the fuzzer is running without
issues. It doesn't prove that all such places are fixed, but I don't
know a better way to test this.