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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 16 Jan 2020 10:18:25 +0100
Message-ID: <CACT4Y+brqD-o-u3Vt=C-PBiS2Wz+wXN3Q3RqBhf3XyRYaRoZJw@mail.gmail.com>
Subject: Re: [RFC PATCH] UML: add support for KASAN under x86_64
To:     Johannes Berg <johannes@sipsolutions.net>
Cc:     Patricia Alfonso <trishalfonso@google.com>,
        Richard Weinberger <richard@nod.at>,
        Jeff Dike <jdike@addtoit.com>,
        Brendan Higgins <brendanhiggins@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        linux-um@lists.infradead.org, David Gow <davidgow@google.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        anton.ivanov@cambridgegreys.com
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On Thu, Jan 16, 2020 at 9:03 AM Johannes Berg <johannes@sipsolutions.net> wrote:
>
> On Thu, 2020-01-16 at 08:57 +0100, Johannes Berg wrote:
> >
> > And if I remember from looking at KASAN, some of the constructors there
> > depended on initializing after the KASAN data structures were set up (or
> > at least allocated)? It may be that you solved that by allocating the
> > shadow so very early though.
>
> Actually, no ... it's still after main(), and the constructors run
> before.
>
> So I _think_ with the CONFIG_CONSTRUCTORS revert, this will no longer
> work (but happy to be proven wrong!), if so then I guess we do have to
> find a way to initialize the KASAN things from another (somehow
> earlier?) constructor ...
>
> Or find a way to fix CONFIG_CONSTRUCTORS and not revert, but I looked at
> it quite a bit and didn't.

Looking at this problem and at the number of KASAN_SANITIZE := n in
Makefiles (some of which are pretty sad, e.g. ignoring string.c,
kstrtox.c, vsprintf.c -- that's where the bugs are!), I think we
initialize KASAN too late. I think we need to do roughly what we do in
user-space asan (because it is user-space asan!). Constructors run
before main and it's really good, we need to initialize KASAN from
these constructors. Or if that's not enough in all cases, also add own
constructor/.preinit array entry to initialize as early as possible.
All we need to do is to call mmap syscall, there is really no
dependencies on anything kernel-related.
This should resolve the problem with constructors (after they
initialize KASAN, they can proceed to do anything they need) and it
should get rid of most KASAN_SANITIZE (in particular, all of
lib/Makefile and kernel/Makefile) and should fix stack instrumentation
(in case it does not work now). The only tiny bit we should not
instrument is the path from constructor up to mmap call.