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

From: Ard Biesheuvel
Date: Thu Nov 26 2015 - 11:41:09 EST

On 26 November 2015 at 14:14, Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> 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>

I would argue that the Kasan code is complicated enough, and we should
avoid complicating it even further for a configuration that is highly
theoretical in nature.

In a 16k configuration, the 4th level only adds a single bit of VA
space (which is, as I understand it, exactly the issue you need to
address here since the top level page table has only 2 entries and
hence does not divide by 8 cleanly), which means you are better off
using 3 levels unless you *really* need more than 128 TB of VA space.

So can't we just live with the limitation, and keep the current code?

--
Ard.

> ---
>
> *** 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));
> kasan_map_early_shadow();
> }
>
> -static void __init clear_pgds(unsigned long start,
> - unsigned long end)
> +static void __init clear_pmds(pud_t *pud, unsigned long addr, unsigned long end)
> {
> + pmd_t *pmd;
> + unsigned long next;
> +
> + pmd = pmd_offset(pud, addr);
> +
> + do {
> + next = pmd_addr_end(addr, end);
> + if (IS_ALIGNED(addr, PMD_SIZE) && end - addr >= PMD_SIZE)
> + pmd_clear(pmd);
> +
> + } while (pmd++, addr = next, addr != end);
> +}
> +
> +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);
> +
> + if (!pud_none(*pud))
> + clear_pmds(pud, addr, next);
> + } while (pud++, addr = next, addr != end);
> +}
> +
> +static void __init clear_page_tables(unsigned long addr, unsigned long end)
> +{
> + pgd_t *pgd;
> + unsigned long next;
> +
> + pgd = pgd_offset_k(addr);
> +
> + do {
> + next = pgd_addr_end(addr, end);
> + if (IS_ALIGNED(addr, PGDIR_SIZE) && end - addr >= PGDIR_SIZE)
> + pgd_clear(pgd);
> +
> + if (!pgd_none(*pgd))
> + clear_puds(pgd, addr, next);
> + } while (pgd++, addr = next, addr != end);
> +}
> +
> +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);
> + }
> }
>
> 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();
>
> - clear_pgds(KASAN_SHADOW_START, KASAN_SHADOW_END);
> + clear_page_tables(KASAN_SHADOW_START, KASAN_SHADOW_END);
>
> kasan_populate_zero_shadow((void *)KASAN_SHADOW_START,
> kasan_mem_to_shadow((void *)MODULES_VADDR));
> --
> 2.4.10
>
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