Re: [RFC PATCH 05/14] khwasan: initialize shadow to 0xff

[Evgenii Stepanov <eugenis@google.com>]

If this memset has noticeable performance/memory impact, we could
treat memory tags as bitwise negation of pointer tags, and then shadow
would be initialized to 0 instead of 0xff.

On Fri, Mar 2, 2018 at 11:44 AM, Andrey Konovalov <andreyknvl@google.com> wrote:
> A KHWASAN shadow memory cell contains a memory tag, that corresponds to
> the tag in the top byte of the pointer, that points to that memory. The
> native top byte value of kernel pointers is 0xff, so with KHWASAN we
> need to initialize shadow memory to 0xff. This commit does that.
> ---
>  arch/arm64/mm/kasan_init.c | 11 ++++++++++-
>  include/linux/kasan.h      |  8 ++++++++
>  mm/kasan/common.c          |  7 +++++++
>  3 files changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
> index dabfc1ecda3d..d4bceba60010 100644
> --- a/arch/arm64/mm/kasan_init.c
> +++ b/arch/arm64/mm/kasan_init.c
> @@ -90,6 +90,10 @@ static void __init kasan_pte_populate(pmd_t *pmdp, unsigned long addr,
>         do {
>                 phys_addr_t page_phys = early ? __pa_symbol(kasan_zero_page)
>                                               : kasan_alloc_zeroed_page(node);
> +#if KASAN_SHADOW_INIT != 0
> +               if (!early)
> +                       memset(__va(page_phys), KASAN_SHADOW_INIT, PAGE_SIZE);
> +#endif
>                 next = addr + PAGE_SIZE;
>                 set_pte(ptep, pfn_pte(__phys_to_pfn(page_phys), PAGE_KERNEL));
>         } while (ptep++, addr = next, addr != end && pte_none(READ_ONCE(*ptep)));
> @@ -139,6 +143,11 @@ asmlinkage void __init kasan_early_init(void)
>                 KASAN_SHADOW_END - (1UL << (64 - KASAN_SHADOW_SCALE_SHIFT)));
>         BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_START, PGDIR_SIZE));
>         BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_END, PGDIR_SIZE));
> +
> +#if KASAN_SHADOW_INIT != 0
> +       memset(kasan_zero_page, KASAN_SHADOW_INIT, PAGE_SIZE);
> +#endif
> +
>         kasan_pgd_populate(KASAN_SHADOW_START, KASAN_SHADOW_END, NUMA_NO_NODE,
>                            true);
>  }
> @@ -235,7 +244,7 @@ void __init kasan_init(void)
>                 set_pte(&kasan_zero_pte[i],
>                         pfn_pte(sym_to_pfn(kasan_zero_page), PAGE_KERNEL_RO));
>
> -       memset(kasan_zero_page, 0, PAGE_SIZE);
> +       memset(kasan_zero_page, KASAN_SHADOW_INIT, PAGE_SIZE);
>         cpu_replace_ttbr1(lm_alias(swapper_pg_dir));
>
>         /* At this point kasan is fully initialized. Enable error messages */
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 3c45e273a936..c34f413b0eac 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -139,6 +139,8 @@ static inline size_t kasan_metadata_size(struct kmem_cache *cache) { return 0; }
>
>  #ifdef CONFIG_KASAN_CLASSIC
>
> +#define KASAN_SHADOW_INIT 0
> +
>  void kasan_cache_shrink(struct kmem_cache *cache);
>  void kasan_cache_shutdown(struct kmem_cache *cache);
>
> @@ -149,4 +151,10 @@ static inline void kasan_cache_shutdown(struct kmem_cache *cache) {}
>
>  #endif /* CONFIG_KASAN_CLASSIC */
>
> +#ifdef CONFIG_KASAN_TAGS
> +
> +#define KASAN_SHADOW_INIT 0xff
> +
> +#endif /* CONFIG_KASAN_TAGS */
> +
>  #endif /* LINUX_KASAN_H */
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 08f6c8cb9f84..f4ccb9425655 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -253,6 +253,9 @@ int kasan_module_alloc(void *addr, size_t size)
>                         __builtin_return_address(0));
>
>         if (ret) {
> +#if KASAN_SHADOW_INIT != 0
> +               __memset(ret, KASAN_SHADOW_INIT, shadow_size);
> +#endif
>                 find_vm_area(addr)->flags |= VM_KASAN;
>                 kmemleak_ignore(ret);
>                 return 0;
> @@ -297,6 +300,10 @@ static int __meminit kasan_mem_notifier(struct notifier_block *nb,
>                 if (!ret)
>                         return NOTIFY_BAD;
>
> +#if KASAN_SHADOW_INIT != 0
> +               __memset(ret, KASAN_SHADOW_INIT, shadow_end - shadow_start);
> +#endif
> +
>                 kmemleak_ignore(ret);
>                 return NOTIFY_OK;
>         }
> --
> 2.16.2.395.g2e18187dfd-goog
>

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