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

Will Deacon
2021-02-04T12:46:38Z

On Sat, Jan 09, 2021 at 06:32:49PM +0800, Lecopzer Chen 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@mediatek.com>
> ---
> 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);
> +
> /*
> * 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)) {

Do we really need yet another CONFIG option for KASAN? What's the use-case
for *not* enabling this if you're already enabling one of the KASAN
backends?

> + kasan_populate_early_shadow((void *)vmalloc_shadow_end,
> + (void *)KASAN_SHADOW_END);
> + if (vmalloc_shadow_start > mod_shadow_end)

To echo Ard's concern: when is the above 'if' condition true?

Will