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Date: Thu, 26 Nov 2015 16:21:17 +0000
From: Mark Rutland <mark.rutland@....com>
To: Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc: Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will.deacon@....com>,
linux-arm-kernel@...ts.infradead.org, Yury <yury.norov@...il.com>,
Alexey Klimov <klimov.linux@...il.com>,
Arnd Bergmann <arnd@...db.de>, linux-mm@...ck.org,
Linus Walleij <linus.walleij@...aro.org>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
linux-kernel@...r.kernel.org,
David Keitel <dkeitel@...eaurora.org>,
Alexander Potapenko <glider@...gle.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
"Suzuki K. Poulose" <Suzuki.Poulose@....com>
Subject: Re: [PATCH RFT] arm64: kasan: Make KASAN work with 16K pages + 48
bit VA
On Thu, Nov 26, 2015 at 06:47:36PM +0300, Andrey Ryabinin wrote:
>
>
> On 11/26/2015 05:48 PM, Mark Rutland wrote:
> > Hi,
> >
> > On Thu, Nov 26, 2015 at 04:14:46PM +0300, Andrey Ryabinin wrote:
> >> Currently kasan assumes that shadow memory covers one or more entire PGDs.
> >> That's not true for 16K pages + 48bit VA space, where PGDIR_SIZE is bigger
> >> than the whole shadow memory.
> >>
> >> This patch tries to fix that case.
> >> clear_page_tables() is a new replacement of clear_pgs(). Instead of always
> >> clearing pgds it clears top level page table entries that entirely belongs
> >> to shadow memory.
> >> In addition to 'tmp_pg_dir' we now have 'tmp_pud' which is used to store
> >> puds that now might be cleared by clear_page_tables.
> >>
> >> Reported-by: Suzuki K. Poulose <Suzuki.Poulose@....com>
> >> Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
> >> ---
> >>
> >> *** THIS is not tested with 16k pages ***
> >>
> >> arch/arm64/mm/kasan_init.c | 87 ++++++++++++++++++++++++++++++++++++++++------
> >> 1 file changed, 76 insertions(+), 11 deletions(-)
> >>
> >> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
> >> index cf038c7..ea9f92a 100644
> >> --- a/arch/arm64/mm/kasan_init.c
> >> +++ b/arch/arm64/mm/kasan_init.c
> >> @@ -22,6 +22,7 @@
> >> #include <asm/tlbflush.h>
> >>
> >> static pgd_t tmp_pg_dir[PTRS_PER_PGD] __initdata __aligned(PGD_SIZE);
> >> +static pud_t tmp_pud[PAGE_SIZE/sizeof(pud_t)] __initdata __aligned(PAGE_SIZE);
> >>
> >> static void __init kasan_early_pte_populate(pmd_t *pmd, unsigned long addr,
> >> unsigned long end)
> >> @@ -92,20 +93,84 @@ asmlinkage void __init kasan_early_init(void)
> >> {
> >> BUILD_BUG_ON(KASAN_SHADOW_OFFSET != KASAN_SHADOW_END - (1UL << 61));
> >> BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_START, PGDIR_SIZE));
> >> - BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_END, PGDIR_SIZE));
> >> + BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_END, PUD_SIZE));
> >
> > We also assume that even in the shared PUD case, the shadow region falls
> > within the same PGD entry, or we would need more than a single tmp_pud.
> >
> > It would be good to test for that.
> >
>
> Something like this:
>
> #define KASAN_SHADOW_SIZE (KASAN_SHADOW_END - KASAN_SHADOW_START)
>
> BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_END, PGD_SIZE)
> && !((PGDIR_SIZE > KASAN_SHADOW_SIZE)
> && IS_ALIGNED(KASAN_SHADOW_END, PUD_SIZE)));
I was thinking something more like:
BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_END, PUD_SIZE);
BUILD_BUG_ON(KASAN_SHADOW_START >> PGDIR_SHIFT !=
KASAN_SHADOW_END >> PGDIR_SHIFT);
> >> + if (!pud_none(*pud))
> >> + clear_pmds(pud, addr, next);
> >
> > I don't understand this. The KASAN shadow region is PUD_SIZE aligned at
> > either end, so KASAN should never own a partial pud entry like this.
> >
> > Regardless, were this case to occur, surely we'd be clearing pmd entries
> > in the active page tables? We didn't copy anything at the pmd level.
> >
> > That doesn't seem right.
> >
>
> Just take a look at p?d_clear() macroses, under CONFIG_PGTABLE_LEVELS=2 for example.
> pgd_clear() and pud_clear() is nops, and pmd_clear() is actually clears pgd.
I see. Thanks for pointing that out.
I detest the weird folding behaviour we have in the p??_* macros. It
violates least surprise almost every time.
> I could replace p?d_clear() with set_p?d(p?d, __p?d(0)).
> In that case going down to pmds is not needed, set_p?d() macro will do it for us.
I think it would be simpler to rely on the fact that we only use puds
with 4 levels of table (and hence the p??_* macros will operate at the
levels their names imply).
We can verify that at build time with:
BUILD_BUG_ON(CONFIG_PGTABLE_LEVELS != 4 &&
(!IS_ALIGNED(KASAN_SHADOW_START, PGDIR_SIZE) ||
!IS_ALIGNED(KASAN_SHADOW_END, PGDIR_SIZE)));
> >> +static void copy_pagetables(void)
> >> +{
> >> + pgd_t *pgd = tmp_pg_dir + pgd_index(KASAN_SHADOW_START);
> >> +
> >> + memcpy(tmp_pg_dir, swapper_pg_dir, sizeof(tmp_pg_dir));
> >> +
> >> /*
> >> - * Remove references to kasan page tables from
> >> - * swapper_pg_dir. pgd_clear() can't be used
> >> - * here because it's nop on 2,3-level pagetable setups
> >> + * If kasan shadow shares PGD with other mappings,
> >> + * clear_page_tables() will clear puds instead of pgd,
> >> + * so we need temporary pud table to keep early shadow mapped.
> >> */
> >> - for (; start < end; start += PGDIR_SIZE)
> >> - set_pgd(pgd_offset_k(start), __pgd(0));
> >> + if (PGDIR_SIZE > KASAN_SHADOW_END - KASAN_SHADOW_START) {
> >> + pud_t *pud;
> >> + pmd_t *pmd;
> >> + pte_t *pte;
> >> +
> >> + memcpy(tmp_pud, pgd_page_vaddr(*pgd), sizeof(tmp_pud));
> >> +
> >> + pgd_populate(&init_mm, pgd, tmp_pud);
> >> + pud = pud_offset(pgd, KASAN_SHADOW_START);
> >> + pmd = pmd_offset(pud, KASAN_SHADOW_START);
> >> + pud_populate(&init_mm, pud, pmd);
> >> + pte = pte_offset_kernel(pmd, KASAN_SHADOW_START);
> >> + pmd_populate_kernel(&init_mm, pmd, pte);
> >
> > I don't understand why we need to do anything below the pud level here.
> > We only copy down to the pud level, and we already initialised the
> > shared ptes and pmds earlier.
> >
> > Regardless of this patch, we currently initialise the shared tables
> > repeatedly, which is redundant after the first time we initialise them.
> > We could improve that.
> >
>
> Sure, just pgd_populate() will work here, because this code is only for 16K+48-bit,
> which has 4-level pagetables.
> But it wouldn't work if 16k+48-bit would have > 4-level.
> Because pgd_populate() in nop in such case, so we need to go down to actually set 'tmp_pud'
I don't follow.
16K + 48-bit will always require 4 levels given the page table format.
We never have more than 4 levels.
Thanks,
Mark.
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