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

[Laura Abbott <labbott@xxxxxxxxxx>]

On 02/25/2016 06:53 PM, Jianyu Zhan wrote:
> 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.

Do you have some suggestion on wording here? I'm not sure what else to
say besides poison patterns to differentiate from hardware poison.
 
> > +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.
 
> Thanks,
> Jianyu Zhan

Thanks,
Laura

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