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

From: Dmitry Vyukov <dvyukov@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Julien Thierry <julien.thierry@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Andy Lutomirski <luto@amacapital.net>,
	Ingo Molnar <mingo@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	James Morse <james.morse@arm.com>,
	valentin.schneider@arm.com, Brian Gerst <brgerst@gmail.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Alexander Potapenko <glider@google.com>
Subject: Re: [PATCH 5/6] objtool: Add UACCESS validation
Date: Thu, 28 Feb 2019 11:05:10 +0100	[thread overview]
Message-ID: <CACT4Y+Y6yp+1fuOMM9xK2gzKk1tPvarjQuUaNC1h0L8EZKenQQ@mail.gmail.com> (raw)
In-Reply-To: <CACT4Y+YSi_XnjNnOhtL1ctQQuZViarxvN+muB-oCmakjgB9mzQ@mail.gmail.com>

On Thu, Feb 28, 2019 at 10:59 AM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Thu, Feb 28, 2019 at 10:40 AM Peter Zijlstra <peterz@infradead.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@infradead.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.

Maybe asm directive will help to trick the compiler?