PAX_MEMORY_SANITIZE + PAGE_POISONING bug

[metarox]

I'm hitting the BUG_ON in the code below at kernel mm_init on ARM kernel 3.4 where page poisoning is enabled from DEBUG_PAGEALLOC (put's 0xAA in all pages when it frees them which is comparable to PAX_MEMORY_SANITIZE). The pages are poisoned from early allocation of the free pages lists at boot.

Code: Select all

static int ioremap_pte_range(pmd_t *pmd, unsigned long addr,
      unsigned long end, phys_addr_t phys_addr, pgprot_t prot)
{
   pte_t *pte;
   u64 pfn;

   pfn = phys_addr >> PAGE_SHIFT;
   pte = pte_alloc_kernel(pmd, addr);
   if (!pte)
      return -ENOMEM;
   do {
      BUG_ON(!pte_none(*pte));
      set_pte_at(&init_mm, addr, pte, pfn_pte(pfn, prot));
      pfn++;
   } while (pte++, addr += PAGE_SIZE, addr != end);
   return 0;
}


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static inline pte_t *
pte_alloc_one_kernel(struct mm_struct *mm, unsigned long addr)
{
   pte_t *pte;

   pte = (pte_t *)__get_free_page(PGALLOC_GFP);
   if (pte)
      clean_pte_table(pte);

   return pte;
}


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#define PGALLOC_GFP   (GFP_KERNEL | __GFP_NOTRACK | __GFP_REPEAT | __GFP_ZERO)


In page_alloc.c this gets called when allocating the page and the fact that it's not zero'd and poisoned means that the pte value in the ioremap call is not zero'd which triggers the BUG_ON.

Code: Select all

static int prep_new_page(struct page *page, int order, gfp_t gfp_flags)
{
   int i;

   for (i = 0; i < (1 << order); i++) {
      struct page *p = page + i;
      if (unlikely(check_new_page(p)))
         return 1;
   }

   set_page_private(page, 0);
   set_page_refcounted(page);

   arch_alloc_page(page, order);
   kernel_map_pages(page, 1 << order, 1);

#ifdef CONFIG_PAX_MEMORY_SANITIZE
   if (gfp_flags & __GFP_ZERO)
      prep_zero_page(page, order, gfp_flags);
#endif

   if (order && (gfp_flags & __GFP_COMP))
      prep_compound_page(page, order);

   return 0;
}


Changing the config ifdef

#ifdef CONFIG_PAX_MEMORY_SANITIZE

to

#if !defined(CONFIG_PAX_MEMORY_SANITIZE) || defined(CONFIG_DEBUG_PAGEALLOC)

fixes the issue for me

Let me know what you think.

[PaX Team]

the fundamental problem is that both of these features want to control the content of free memory pages and are therefore incompatible with each other. your change 'fixes' the symptom by undoing a performance optimization enabled by SANITIZE (early zeroing makes it unnecessary to do it again on allocation) but by that point the early zeroing was already long undone and hence unnecessary to begin with. so i'll just disable SANITIZE when DEBUG_PAGEALLOC is enabled in .config.

[metarox]

This triggers a recursive dependency Kconfig error in my attempt

kernel/power/Kconfig:24:error: recursive dependency detected!
kernel/power/Kconfig:24: symbol HIBERNATION depends on PAX_MEMORY_SANITIZE
security/Kconfig:743: symbol PAX_MEMORY_SANITIZE depends on DEBUG_PAGEALLOC
mm/Kconfig.debug:1: symbol DEBUG_PAGEALLOC depends on HIBERNATION

Let me know what you use for the fix and I can try it out.

[PaX Team]

well, PaX and grsec diverge on option dependencies sometimes as spender prefers to disable the vanilla feature instead of the PaX one so in your case you'd have to make DEBUG_PAGEALLOC depend on !PAX_MEMORY_SANITIZE much like HIBERNATION does.