linux-kernel - Re: [RFC PATCH 1/2] lib/string: Disable instrumentation

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Date:   Tue, 8 Sep 2020 14:40:03 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Kees Cook <keescook@...omium.org>
Cc:     Marco Elver <elver@...gle.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        the arch/x86 maintainers <x86@...nel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [RFC PATCH 1/2] lib/string: Disable instrumentation

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@...m.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.