Re: KASAN vs. boot-time switching between 4- and 5-level paging

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>,
	Alexander Potapenko <glider@google.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"x86@kernel.org" <x86@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Andi Kleen <ak@linux.intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Andy Lutomirski <luto@amacapital.net>,
	linux-arch@vger.kernel.org,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kasan-dev <kasan-dev@googlegroups.com>
Subject: Re: KASAN vs. boot-time switching between 4- and 5-level paging
Date: Mon, 10 Jul 2017 15:33:46 +0300	[thread overview]
Message-ID: <20170710123346.7y3jnftqgpingim3@node.shutemov.name> (raw)
In-Reply-To: <bc95be68-8c68-2a45-c530-acbc6c90a231@virtuozzo.com>

On Thu, Jun 01, 2017 at 05:56:30PM +0300, Andrey Ryabinin wrote:
> On 05/29/2017 03:46 PM, Andrey Ryabinin wrote:
> > On 05/29/2017 02:45 PM, Andrey Ryabinin wrote:
> >>>>>> Looks like KASAN will be a problem for boot-time paging mode switching.
> >>>>>> It wants to know CONFIG_KASAN_SHADOW_OFFSET at compile-time to pass to
> >>>>>> gcc -fasan-shadow-offset=. But this value varies between paging modes...
> >>>>>>
> >>>>>> I don't see how to solve it. Folks, any ideas?
> >>>>>
> >>>>> +kasan-dev
> >>>>>
> >>>>> I wonder if we can use the same offset for both modes. If we use
> >>>>> 0xFFDFFC0000000000 as start of shadow for 5 levels, then the same
> >>>>> offset that we use for 4 levels (0xdffffc0000000000) will also work
> >>>>> for 5 levels. Namely, ending of 5 level shadow will overlap with 4
> >>>>> level mapping (both end at 0xfffffbffffffffff), but 5 level mapping
> >>>>> extends towards lower addresses. The current 5 level start of shadow
> >>>>> is actually close -- 0xffd8000000000000 and it seems that the required
> >>>>> space after it is unused at the moment (at least looking at mm.txt).
> >>>>> So just try to move it to 0xFFDFFC0000000000?
> >>>>>
> >>>>
> >>>> Yeah, this should work, but note that 0xFFDFFC0000000000 is not PGDIR aligned address. Our init code
> >>>> assumes that kasan shadow stars and ends on the PGDIR aligned address.
> >>>> Fortunately this is fixable, we'd need two more pages for page tables to map unaligned start/end
> >>>> of the shadow.
> >>>
> >>> I think we can extend the shadow backwards (to the current address),
> >>> provided that it does not affect shadow offset that we pass to
> >>> compiler.
> >>
> >> I thought about this. We can round down shadow start to 0xffdf000000000000, but we can't
> >> round up shadow end, because in that case shadow would end at 0xffffffffffffffff.
> >> So we still need at least one more page to cover unaligned end.
> > 
> > Actually, I'm wrong here. I assumed that we would need an additional page to store p4d entries,
> > but in fact we don't need it, as such page should already exist. It's the same last pgd where kernel image
> > is mapped.
> > 
> 
> 
> Something like bellow might work. It's just a proposal to demonstrate the idea, so some code might look ugly.
> And it's only build-tested.

[Sorry for loong delay.]

The patch works for me for legacy boot. But it breaks EFI boot with
5-level paging. And I struggle to understand why.

What I see is many page faults at mm/kasan/kasan.c:758 --
"DEFINE_ASAN_LOAD_STORE(4)". Handling one of them I get double-fault at
arch/x86/kernel/head_64.S:298 -- "pushq %r14", which ends up with triple
fault.

Any ideas?

If you want to play with this by yourself, qemu supports la57 -- use
-cpu "qemu64,+la57".

-- 
 Kirill A. Shutemov

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