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 <[email protected]>
>> Signed-off-by: Andrey Ryabinin <[email protected]>
>> ---
>>
>> *** 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)));
>> +static void __init clear_puds(pgd_t *pgd, unsigned long addr, unsigned long
>> end)
>> +{
>> + pud_t *pud;
>> + unsigned long next;
>> +
>> + pud = pud_offset(pgd, addr);
>> +
>> + do {
>> + next = pud_addr_end(addr, end);
>> + if (IS_ALIGNED(addr, PUD_SIZE) && end - addr >= PUD_SIZE)
>> + pud_clear(pud);
>
> I think this can just be:
>
> if (next - addr == PUD_SIZE)
> pud_clear(pud);
>
> Given that next can at most be PUD_SIZE from addr, and if so we knwo
> addr is aligned.
>
Right.
>> +
>> + 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 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.
...
>> +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 just tried to avoid assumptions about number of pagetables levels in that
code.
>> + }
>> }
>>
>> static void __init cpu_set_ttbr1(unsigned long ttbr1)
>> @@ -123,16 +188,16 @@ void __init kasan_init(void)
>>
>> /*
>> * We are going to perform proper setup of shadow memory.
>> - * At first we should unmap early shadow (clear_pgds() call bellow).
>> + * At first we should unmap early shadow (clear_page_tables()).
>> * However, instrumented code couldn't execute without shadow memory.
>> * tmp_pg_dir used to keep early shadow mapped until full shadow
>> * setup will be finished.
>> */
>> - memcpy(tmp_pg_dir, swapper_pg_dir, sizeof(tmp_pg_dir));
>> + copy_pagetables();
>> cpu_set_ttbr1(__pa(tmp_pg_dir));
>> flush_tlb_all();
>
> As a heads-up, I will shortly have patches altering the swap of TTBR1,
> as it risks conflicting TLB entries and misses barriers.
>
> Otherwise, we need a dsb(ishst) between the memcpy and writing to the
> TTBR to ensure that the tables are visible to the MMU.
>
Thanks.
> Thanks,
> Mark.
>
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