linux-kernel - Re: [PATCH 5/6] objtool: Add UACCESS validation

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Date:   Thu, 28 Feb 2019 10:40:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Julien Thierry <julien.thierry@....com>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>, valentin.schneider@....com,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH 5/6] objtool: Add UACCESS validation

On Wed, Feb 27, 2019 at 06:28:16PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 27, 2019 at 04:40:28PM +0100, Dmitry Vyukov wrote:
> > On Wed, Feb 27, 2019 at 3:33 PM Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > > Urgh, kasan_report() is definitely unsafe. Now, admitedly we should
> > > 'never' hit that, but it does leave us up a creek without a paddle.
> 
> > If SMAP detects additional bugs, then it would be pity to disable it
> > with KASAN (detect bugs in production but not during testing).
> > 
> > You mentioned that exception save/restore the UACCESS state. Is it
> > possible to do the same in kasan_report? At the very least we need to
> > survive report printing, what happens after that does not matter much
> > (we've corrupted memory by now anyway).
> 
> Ideally we'll put all of kasan_report() in an exception, much like we do
> for WARN. But there's a distinct lack of arch hooks there to play with.
> I suppose I can try and create some.
> 
> On top of that we'll have to mark these __asan functions with notrace.
> 
> Maybe a little something horrible like so... completely untested.

OK, I got that to compile; the next problem is:

../include/linux/kasan.h:90:1: error: built-in function ‘__asan_loadN_noabort’ must be directly called
UACCESS_SAFE(__asan_loadN_noabort);

Which doesn't make any sense; since we actually generated that symbol,
it clearly is not built-in. What gives?