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

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Date:   Thu, 28 Feb 2019 10:59:10 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 Thu, Feb 28, 2019 at 10:40 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> 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?

I guess this warning originated for user-space where programmer does
not define them and does not generally know about them and signature
is not a public contract for user. And then for kernel it just stayed
the same because not doing this warning would require somebody to
proactively think about this potential difference and add an
additional code to skip this check and even then it wasn't obvious why
one will want to do this with these functions. So that's where we are
now.