Re: [PATCH RFT] arm64: kasan: Make KASAN work with 16K pages + 48 bit VA

[Mark Rutland]

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@xxxxxxx>
> >> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> >> ---
> >>
> >>  *** 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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