Re: [RFC][PATCH v3 1/2] mm/page_poison.c: Enable PAGE_POISONING as a separate option

[Jianyu Zhan]

On Thu, Feb 25, 2016 at 7:35 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>  config PAGE_POISONING
> -       bool
> +       bool "Poison pages after freeing"
> +       select PAGE_EXTENSION
> +       select PAGE_POISONING_NO_SANITY if HIBERNATION
> +       ---help---
> +         Fill the pages with poison patterns after free_pages() and verify
> +         the patterns before alloc_pages. The filling of the memory helps
> +         reduce the risk of information leaks from freed data. This does
> +         have a potential performance impact.
> +
> +         If unsure, say N
> +

I would suggest that you add some wording in the help text to clarify
that what "poisoning"
means here is not the same as that in "HWPoison".

The previous one is pattern padding, while the latter one is just
nomenclature borrowed from
Intel for memory failure.

> +config PAGE_POISONING_NO_SANITY
> +       depends on PAGE_POISONING
> +       bool "Only poison, don't sanity check"
> +       ---help---
> +          Skip the sanity checking on alloc, only fill the pages with
> +          poison on free. This reduces some of the overhead of the
> +          poisoning feature.
> +
> +          If you are only interested in sanitization, say Y. Otherwise
> +          say N.
> diff --git a/mm/Makefile b/mm/Makefile
> index fb1a7948c107..ec59c071b4f9 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -13,7 +13,6 @@ KCOV_INSTRUMENT_slob.o := n
>  KCOV_INSTRUMENT_slab.o := n
>  KCOV_INSTRUMENT_slub.o := n
>  KCOV_INSTRUMENT_page_alloc.o := n
> -KCOV_INSTRUMENT_debug-pagealloc.o := n
>  KCOV_INSTRUMENT_kmemleak.o := n
>  KCOV_INSTRUMENT_kmemcheck.o := n
>  KCOV_INSTRUMENT_memcontrol.o := n
> @@ -63,9 +62,6 @@ obj-$(CONFIG_SPARSEMEM_VMEMMAP) += sparse-vmemmap.o
>  obj-$(CONFIG_SLOB) += slob.o
>  obj-$(CONFIG_MMU_NOTIFIER) += mmu_notifier.o
>  obj-$(CONFIG_KSM) += ksm.o
> -ifndef CONFIG_ARCH_SUPPORTS_DEBUG_PAGEALLOC
> -       obj-$(CONFIG_DEBUG_PAGEALLOC) += debug-pagealloc.o
> -endif
>  obj-$(CONFIG_PAGE_POISONING) += page_poison.o
>  obj-$(CONFIG_SLAB) += slab.o
>  obj-$(CONFIG_SLUB) += slub.o
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index a34c359d8e81..0bdb3cfd83b5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1026,6 +1026,7 @@ static bool free_pages_prepare(struct page *page, unsigned int order)
>                                            PAGE_SIZE << order);
>         }
>         arch_free_page(page, order);
> +       kernel_poison_pages(page, 1 << order, 0);
>         kernel_map_pages(page, 1 << order, 0);
>
>         return true;
> @@ -1497,6 +1498,7 @@ static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
>
>         arch_alloc_page(page, order);
>         kernel_map_pages(page, 1 << order, 1);
> +       kernel_poison_pages(page, 1 << order, 1);
>         kasan_alloc_pages(page, order);
>
>         if (gfp_flags & __GFP_ZERO)
> diff --git a/mm/page_poison.c b/mm/page_poison.c
> index 92ead727b8f0..884a6f854432 100644
> --- a/mm/page_poison.c
> +++ b/mm/page_poison.c
> @@ -80,7 +80,7 @@ static void poison_page(struct page *page)
>         kunmap_atomic(addr);
>  }
>
> -void poison_pages(struct page *page, int n)
> +static void poison_pages(struct page *page, int n)
>  {
>         int i;
>
> @@ -101,6 +101,9 @@ static void check_poison_mem(unsigned char *mem, size_t bytes)
>         unsigned char *start;
>         unsigned char *end;
>
> +       if (IS_ENABLED(CONFIG_PAGE_POISONING_NO_SANITY))
> +               return;
> +
>         start = memchr_inv(mem, PAGE_POISON, bytes);
>         if (!start)
>                 return;
> @@ -113,9 +116,9 @@ static void check_poison_mem(unsigned char *mem, size_t bytes)
>         if (!__ratelimit(&ratelimit))
>                 return;
>         else if (start == end && single_bit_flip(*start, PAGE_POISON))
> -               printk(KERN_ERR "pagealloc: single bit error\n");
> +               pr_err("pagealloc: single bit error\n");
>         else
> -               printk(KERN_ERR "pagealloc: memory corruption\n");
> +               pr_err("pagealloc: memory corruption\n");
>
>         print_hex_dump(KERN_ERR, "", DUMP_PREFIX_ADDRESS, 16, 1, start,
>                         end - start + 1, 1);
> @@ -135,10 +138,28 @@ static void unpoison_page(struct page *page)
>         kunmap_atomic(addr);
>  }
>
> -void unpoison_pages(struct page *page, int n)
> +static void unpoison_pages(struct page *page, int n)
>  {
>         int i;
>
>         for (i = 0; i < n; i++)
>                 unpoison_page(page + i);
>  }
> +
> +void kernel_poison_pages(struct page *page, int numpages, int enable)
> +{
> +       if (!page_poisoning_enabled())
> +               return;
> +
> +       if (enable)
> +               unpoison_pages(page, numpages);
> +       else
> +               poison_pages(page, numpages);
> +}
> +
> +#ifndef CONFIG_ARCH_SUPPORTS_DEBUG_PAGEALLOC
> +void __kernel_map_pages(struct page *page, int numpages, int enable)
> +{
> +       /* This function does nothing, all work is done via poison pages */
> +}
> +#endif

IMHO,  kernel_map_pages is originally incorporated for debugging page
allocation.
And latter for archs that do not support arch-specific page poisoning,
a software poisoning
method was used.

So I think it is not appropriate to use two interfaces in the alloc/free hooks.

The kernel_poison_pages actually should be an implementation detail
and should be hided
in the kernel_map_pages interface.


Thanks,
Jianyu Zhan