linux-kernel - Re: [PATCH 1/2] kasan: test: use underlying string helpers

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Date:   Mon, 18 Oct 2021 21:55:51 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-hardening@...r.kernel.org,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marco Elver <elver@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Peter Collingbourne <pcc@...gle.com>,
        Patricia Alfonso <trishalfonso@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] kasan: test: use underlying string helpers

On Mon, Oct 18, 2021 at 9:47 PM Kees Cook <keescook@...omium.org> wrote:
> On Wed, Oct 13, 2021 at 05:00:05PM +0200, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@...db.de>
> >
> > Calling memcmp() and memchr() with an intentional buffer overflow
> > is now caught at compile time:
> >
> > In function 'memcmp',
> >     inlined from 'kasan_memcmp' at lib/test_kasan.c:897:2:
> > include/linux/fortify-string.h:263:25: error: call to '__read_overflow' declared with attribute error: detected read beyond size of object (1st parameter)
> >   263 |                         __read_overflow();
> >       |                         ^~~~~~~~~~~~~~~~~
> > In function 'memchr',
> >     inlined from 'kasan_memchr' at lib/test_kasan.c:872:2:
> > include/linux/fortify-string.h:277:17: error: call to '__read_overflow' declared with attribute error: detected read beyond size of object (1st parameter)
> >   277 |                 __read_overflow();
> >       |                 ^~~~~~~~~~~~~~~~~
> >
> > Change the kasan tests to wrap those inside of a noinline function
> > to prevent the compiler from noticing the bug and let kasan find
> > it at runtime.
>
> Is this with W=1 ? I had explicitly disabled the read overflows for
> "phase 1" of the overflow restriction tightening...

I have a somewhat modified source tree that builds cleanly with W=1 after
disabling all the noisy ones, so this is probably one that I would not have
seen without it.

> (And what do you think of using OPTIMIZER_HIDE_VAR() instead[1]?
>
> [1] https://lore.kernel.org/linux-hardening/20211006181544.1670992-1-keescook@chromium.org/T/#u

Yes, that is probably better. I can try updating the patch tomorrow,
unless you do it first.

       Arnd