Re: [PATCH 3/3] x86/kasan: support KASAN_VMALLOC

[Dmitry Vyukov <dvyukov@google.com>]

On Thu, Jul 25, 2019 at 7:55 AM Daniel Axtens <dja@axtens.net> wrote:
>
> In the case where KASAN directly allocates memory to back vmalloc
> space, don't map the early shadow page over it.
>
> Not mapping the early shadow page over the whole shadow space means
> that there are some pgds that are not populated on boot. Allow the
> vmalloc fault handler to also fault in vmalloc shadow as needed.
>
> Signed-off-by: Daniel Axtens <dja@axtens.net>


Would it make things simpler if we pre-populate the top level page
tables for the whole vmalloc region? That would be
(16<<40)/4096/512/512*8 = 131072 bytes?
The check in vmalloc_fault in not really a big burden, so I am not
sure. Just brining as an option.

Acked-by: Dmitry Vyukov <dvyukov@google.com>

> ---
>  arch/x86/Kconfig            |  1 +
>  arch/x86/mm/fault.c         | 13 +++++++++++++
>  arch/x86/mm/kasan_init_64.c | 10 ++++++++++
>  3 files changed, 24 insertions(+)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 222855cc0158..40562cc3771f 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -134,6 +134,7 @@ config X86
>         select HAVE_ARCH_JUMP_LABEL
>         select HAVE_ARCH_JUMP_LABEL_RELATIVE
>         select HAVE_ARCH_KASAN                  if X86_64
> +       select HAVE_ARCH_KASAN_VMALLOC          if X86_64
>         select HAVE_ARCH_KGDB
>         select HAVE_ARCH_MMAP_RND_BITS          if MMU
>         select HAVE_ARCH_MMAP_RND_COMPAT_BITS   if MMU && COMPAT
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 6c46095cd0d9..d722230121c3 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -340,8 +340,21 @@ static noinline int vmalloc_fault(unsigned long address)
>         pte_t *pte;
>
>         /* Make sure we are in vmalloc area: */
> +#ifndef CONFIG_KASAN_VMALLOC
>         if (!(address >= VMALLOC_START && address < VMALLOC_END))
>                 return -1;
> +#else
> +       /*
> +        * Some of the shadow mapping for the vmalloc area lives outside the
> +        * pgds populated by kasan init. They are created dynamically and so
> +        * we may need to fault them in.
> +        *
> +        * You can observe this with test_vmalloc's align_shift_alloc_test
> +        */
> +       if (!((address >= VMALLOC_START && address < VMALLOC_END) ||
> +             (address >= KASAN_SHADOW_START && address < KASAN_SHADOW_END)))
> +               return -1;
> +#endif
>
>         /*
>          * Copy kernel mappings over when needed. This can also
> diff --git a/arch/x86/mm/kasan_init_64.c b/arch/x86/mm/kasan_init_64.c
> index 296da58f3013..e2fe1c1b805c 100644
> --- a/arch/x86/mm/kasan_init_64.c
> +++ b/arch/x86/mm/kasan_init_64.c
> @@ -352,9 +352,19 @@ void __init kasan_init(void)
>         shadow_cpu_entry_end = (void *)round_up(
>                         (unsigned long)shadow_cpu_entry_end, PAGE_SIZE);
>
> +       /*
> +        * If we're in full vmalloc mode, don't back vmalloc space with early
> +        * shadow pages.
> +        */
> +#ifdef CONFIG_KASAN_VMALLOC
> +       kasan_populate_early_shadow(
> +               kasan_mem_to_shadow((void *)VMALLOC_END+1),
> +               shadow_cpu_entry_begin);
> +#else
>         kasan_populate_early_shadow(
>                 kasan_mem_to_shadow((void *)PAGE_OFFSET + MAXMEM),
>                 shadow_cpu_entry_begin);
> +#endif
>
>         kasan_populate_shadow((unsigned long)shadow_cpu_entry_begin,
>                               (unsigned long)shadow_cpu_entry_end, 0);
> --
> 2.20.1
>
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