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

[Laura Abbott <labbott@redhat.com>]

On 02/25/2016 09:34 PM, Jianyu Zhan wrote:
> On Fri, Feb 26, 2016 at 12:45 PM, Laura Abbott <labbott@redhat.com> wrote:
>> Do you have some suggestion on wording here? I'm not sure what else to
>> say besides poison patterns to differentiate from hardware poison.
>>
>
>
> Is the below wording OK?
>
>
> 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.
>
>               Note that "poison" here is not the same thing as that in "HWPoison"
>               for CONFIG_MEMORY_FAILURE, in which "poison" is just a nomenclature
>               borrowed from Intel , for the processor support for
> "poisoned" memory, an
>               adaptive method for flagging and recovering from memory errors
>

Okay, I see what you are getting at here. This sounds okay.
  
>>
>>>>
>>>> +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.
>>>
>>
>> We want to have the poisoning independent of anything that kernel_map_pages
>> does. It was originally added for software poisoning for arches that
>> didn't have the full ARCH_SUPPORTS_DEBUG_PAGEALLOC support but there's
>> nothing that specifically ties it to mapping. It's beneficial even when
>> we aren't mapping/unmapping the pages so putting it in kernel_map_pages
>> would defeat what we're trying to accomplish here.
>>
>
> Ok, fair enough. If so,  I suggest you add this clarification into the
> code, or as least, in
> the changelog.

Sounds fine.

>
>
> Thanks,
> Jianyu Zhan
>

Thanks,
Laura

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