Re: [PATCH v2 1/4] arm64: kasan: don't populate vmalloc area for CONFIG_KASAN_VMALLOC

[Ard Biesheuvel]

On Sat, 9 Jan 2021 at 11:33, Lecopzer Chen <lecopzer@xxxxxxxxx> wrote:
>
> Linux support KAsan for VMALLOC since commit 3c5c3cfb9ef4da9
> ("kasan: support backing vmalloc space with real shadow memory")
>
> Like how the MODULES_VADDR does now, just not to early populate
> the VMALLOC_START between VMALLOC_END.
> similarly, the kernel code mapping is now in the VMALLOC area and
> should keep these area populated.
>
> Signed-off-by: Lecopzer Chen <lecopzer.chen@xxxxxxxxxxxx>


This commit log text is a bit hard to follow. You are saying that the
vmalloc region is *not* backed with zero shadow or any default mapping
at all, right, and everything gets allocated on demand, just like is
the case for modules?

> ---
>  arch/arm64/mm/kasan_init.c | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
> index d8e66c78440e..39b218a64279 100644
> --- a/arch/arm64/mm/kasan_init.c
> +++ b/arch/arm64/mm/kasan_init.c
> @@ -214,6 +214,7 @@ static void __init kasan_init_shadow(void)
>  {
>         u64 kimg_shadow_start, kimg_shadow_end;
>         u64 mod_shadow_start, mod_shadow_end;
> +       u64 vmalloc_shadow_start, vmalloc_shadow_end;
>         phys_addr_t pa_start, pa_end;
>         u64 i;
>
> @@ -223,6 +224,9 @@ static void __init kasan_init_shadow(void)
>         mod_shadow_start = (u64)kasan_mem_to_shadow((void *)MODULES_VADDR);
>         mod_shadow_end = (u64)kasan_mem_to_shadow((void *)MODULES_END);
>
> +       vmalloc_shadow_start = (u64)kasan_mem_to_shadow((void *)VMALLOC_START);
> +       vmalloc_shadow_end = (u64)kasan_mem_to_shadow((void *)VMALLOC_END);
> +


This and the below seems overly complicated, given that VMALLOC_START
== MODULES_END. Can we simplify this?

>         /*
>          * We are going to perform proper setup of shadow memory.
>          * At first we should unmap early shadow (clear_pgds() call below).
> @@ -241,12 +245,21 @@ static void __init kasan_init_shadow(void)
>
>         kasan_populate_early_shadow(kasan_mem_to_shadow((void *)PAGE_END),
>                                    (void *)mod_shadow_start);
> -       kasan_populate_early_shadow((void *)kimg_shadow_end,
> -                                  (void *)KASAN_SHADOW_END);
> +       if (IS_ENABLED(CONFIG_KASAN_VMALLOC)) {
> +               kasan_populate_early_shadow((void *)vmalloc_shadow_end,
> +                                           (void *)KASAN_SHADOW_END);
> +               if (vmalloc_shadow_start > mod_shadow_end)
> +                       kasan_populate_early_shadow((void *)mod_shadow_end,
> +                                                   (void *)vmalloc_shadow_start);
> +
> +       } else {
> +               kasan_populate_early_shadow((void *)kimg_shadow_end,
> +                                           (void *)KASAN_SHADOW_END);
> +               if (kimg_shadow_start > mod_shadow_end)
> +                       kasan_populate_early_shadow((void *)mod_shadow_end,
> +                                                   (void *)kimg_shadow_start);
> +       }
>
> -       if (kimg_shadow_start > mod_shadow_end)
> -               kasan_populate_early_shadow((void *)mod_shadow_end,
> -                                           (void *)kimg_shadow_start);
>
>         for_each_mem_range(i, &pa_start, &pa_end) {
>                 void *start = (void *)__phys_to_virt(pa_start);
> --
> 2.25.1
>