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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Wed, 9 Sep 2020 07:20:24 +0200
Message-ID: <CACT4Y+aCa0Y8t198GSwEFShUPuOsqFV5eP8GY_7TK8fi_pML_Q@mail.gmail.com>
Subject: Re: [RFC PATCH 1/2] lib/string: Disable instrumentation
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Kees Cook <keescook@chromium.org>, Marco Elver <elver@google.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrey Konovalov <andreyknvl@google.com>,
        Alexander Potapenko <glider@google.com>
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On Tue, Sep 8, 2020 at 8:40 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> On Tue, Sep 08, 2020 at 10:21:32AM -0700, Kees Cook wrote:
> > On Tue, Sep 08, 2020 at 11:39:11AM +0200, Marco Elver wrote:
> > > On Sun, 6 Sep 2020 at 00:23, Arvind Sankar <nivedita@alum.mit.edu> wrote:
> > > >
> > > > String functions can be useful in early boot, but using instrumented
> > > > versions can be problematic: eg on x86, some of the early boot code is
> > > > executing out of an identity mapping rather than the kernel virtual
> > > > addresses. Accessing any global variables at this point will lead to a
> > > > crash.
> > > >
> > >
> > > Ouch.
> > >
> > > We have found manifestations of bugs in lib/string.c functions, e.g.:
> > >   https://groups.google.com/forum/#!msg/syzkaller-bugs/atbKWcFqE9s/x7AtoVoBAgAJ
> > >   https://groups.google.com/forum/#!msg/syzkaller-bugs/iGBUm-FDhkM/chl05uEgBAAJ
> > >
> > > Is there any way this can be avoided?
> >
> > Agreed: I would like to keep this instrumentation; it's a common place
> > to find bugs, security issues, etc.
> >
> > --
> > Kees Cook
>
> Ok, understood. I'll revise to open-code the strscpy instead.
>
> Is instrumentation supported on x86-32? load_ucode_bsp() on 32-bit is
> called before paging is enabled, and load_ucode_bsp() itself, along with
> eg lib/earlycpio and lib/string that it uses, don't have anything to
> disable instrumentation. kcov, kasan, kcsan are unsupported already on
> 32-bit, but the others like gcov and PROFILE_ALL_BRANCHES look like they
> would just cause a crash if microcode loading is enabled.

I agree we should not disable instrumentation of such common functions.

Instead of open-coding these functions maybe we could produce both
instrumented and non-instrumented versions from the same source
implementation. Namely, place implementation in a header function with
always_inline attribute and include it from 2 source files, one with
instrumentation enabled and another with instrumentation disabled.
This way we could produce strscpy (instrumented) and __strscpy
(non-instrumented) from the same source.