linux-kernel - Re: [PATCH v7 0/4] KASAN for arm64

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Date:	Wed, 18 Nov 2015 17:24:23 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Andrey Ryabinin <ryabinin.a.a@...il.com>
Cc:	linux-arm-kernel@...ts.infradead.org, Yury <yury.norov@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Andrey Konovalov <andreyknvl@...gle.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	"Suzuki K. Poulose" <Suzuki.Poulose@....com>,
	Will Deacon <will.deacon@....com>,
	linux-kernel@...r.kernel.org,
	kasan-dev <kasan-dev@...glegroups.com>, linux-mm@...ck.org,
	Alexander Potapenko <glider@...gle.com>,
	Alexey Klimov <klimov.linux@...il.com>,
	David Keitel <dkeitel@...eaurora.org>,
	Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v7 0/4] KASAN for arm64

On Wed, Nov 18, 2015 at 05:33:43PM +0300, Andrey Ryabinin wrote:
> On 11/16/2015 07:51 PM, Catalin Marinas wrote:
> > On Mon, Nov 16, 2015 at 06:34:27PM +0300, Andrey Ryabinin wrote:
> >> On 11/16/2015 02:16 PM, Suzuki K. Poulose wrote:
> >>> arch/arm64/mm/kasan_init.c:95:2: note: in expansion of macro ‘BUILD_BUG_ON’
> >>>   BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_END, PGDIR_SIZE));
> >>>
> >>> The problem is that the PGDIR_SIZE is (1UL << 47) with 16K+48bit, which makes
> >>> the KASAN_SHADOW_END unaligned(which is aligned to (1UL << (48 - 3)) ). Is the
> >>> alignment really needed ? Thoughts on how best we could fix this ?
> >>
> >> Yes, it's really needed, because some code relies on this (e.g.
> >> clear_pgs() and kasan_init()). But it should be possible to get rid of
> >> this requirement.
> > 
> > I don't think clear_pgds() and kasan_init() are the only problems. IIUC,
> > kasan_populate_zero_shadow() also assumes that KASan shadow covers
> > multiple pgds. You need some kind of recursive writing which avoids
> > populating an entry which is not empty (like kasan_early_pud_populate).
> 
> I think kasan_populate_zero_shadow() should be fine. We call pgd_populate() only
> if address range covers the entire pgd:
> 
> 		if (IS_ALIGNED(addr, PGDIR_SIZE) && end - addr >= PGDIR_SIZE) {
> ....
> 			pgd_populate(&init_mm, pgd, kasan_zero_pud);
> ....
> 
> and otherwise we check for pgd_none(*pgd):
> 		if (pgd_none(*pgd)) {
> 			pgd_populate(&init_mm, pgd,
> 				early_alloc(PAGE_SIZE, NUMA_NO_NODE));
> 		}

OK, I missed the fact that zero_pud_populate() handles the pmd/pte
population with kasan_zero_*.

So if it's only tmp_pg_dir, as you said already, you can add a tmp_pud
for the case where KASAN_SHADOW_SIZE is smaller than PGDIR_SIZE and
change clear_pgds() to erase the puds.

-- 
Catalin
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