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Date: Thu, 28 Jan 2016 20:46:52 -0800
From: Kees Cook <keescook@...omium.org>
To: Laura Abbott <labbott@...oraproject.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Vlastimil Babka <vbabka@...e.cz>,
Michal Hocko <mhocko@...e.com>, "Rafael J. Wysocki" <rjw@...ysocki.net>, Pavel Machek <pavel@....cz>,
Len Brown <len.brown@...el.com>, Linux-MM <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, linux-pm@...r.kernel.org
Subject: Re: [PATCHv2 2/2] mm/page_poisoning.c: Allow for zero poisoning
On Thu, Jan 28, 2016 at 6:38 PM, Laura Abbott <labbott@...oraproject.org> wrote:
> By default, page poisoning uses a poison value (0xaa) on free. If this
> is changed to 0, the page is not only sanitized but zeroing on alloc
> with __GFP_ZERO can be skipped as well. The tradeoff is that detecting
> corruption from the poisoning is harder to detect. This feature also
> cannot be used with hibernation since pages are not guaranteed to be
> zeroed after hibernation.
>
> Credit to Grsecurity/PaX team for inspiring this work
>
> Signed-off-by: Laura Abbott <labbott@...oraproject.org>
> ---
> include/linux/mm.h | 2 ++
> include/linux/poison.h | 4 ++++
> kernel/power/hibernate.c | 17 +++++++++++++++++
> mm/Kconfig.debug | 14 ++++++++++++++
> mm/page_alloc.c | 11 ++++++++++-
> mm/page_ext.c | 10 ++++++++--
> mm/page_poison.c | 7 +++++--
> 7 files changed, 60 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 966bf0e..59ce0dc 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2177,10 +2177,12 @@ extern int apply_to_page_range(struct mm_struct *mm, unsigned long address,
> #ifdef CONFIG_PAGE_POISONING
> extern bool page_poisoning_enabled(void);
> extern void kernel_poison_pages(struct page *page, int numpages, int enable);
> +extern bool page_is_poisoned(struct page *page);
> #else
> static inline bool page_poisoning_enabled(void) { return false; }
> static inline void kernel_poison_pages(struct page *page, int numpages,
> int enable) { }
> +static inline bool page_is_poisoned(struct page *page) { return false; }
> #endif
>
> #ifdef CONFIG_DEBUG_PAGEALLOC
> diff --git a/include/linux/poison.h b/include/linux/poison.h
> index 4a27153..51334ed 100644
> --- a/include/linux/poison.h
> +++ b/include/linux/poison.h
> @@ -30,7 +30,11 @@
> #define TIMER_ENTRY_STATIC ((void *) 0x300 + POISON_POINTER_DELTA)
>
> /********** mm/debug-pagealloc.c **********/
> +#ifdef CONFIG_PAGE_POISONING_ZERO
> +#define PAGE_POISON 0x00
> +#else
> #define PAGE_POISON 0xaa
> +#endif
>
> /********** mm/page_alloc.c ************/
>
> diff --git a/kernel/power/hibernate.c b/kernel/power/hibernate.c
> index b7342a2..aa0f26b 100644
> --- a/kernel/power/hibernate.c
> +++ b/kernel/power/hibernate.c
> @@ -1158,6 +1158,22 @@ static int __init kaslr_nohibernate_setup(char *str)
> return nohibernate_setup(str);
> }
>
> +static int __init page_poison_nohibernate_setup(char *str)
> +{
> +#ifdef CONFIG_PAGE_POISONING_ZERO
> + /*
> + * The zeroing option for page poison skips the checks on alloc.
> + * since hibernation doesn't save free pages there's no way to
> + * guarantee the pages will still be zeroed.
> + */
> + if (!strcmp(str, "on")) {
> + pr_info("Disabling hibernation due to page poisoning\n");
> + return nohibernate_setup(str);
> + }
> +#endif
> + return 1;
> +}
> +
> __setup("noresume", noresume_setup);
> __setup("resume_offset=", resume_offset_setup);
> __setup("resume=", resume_setup);
> @@ -1166,3 +1182,4 @@ __setup("resumewait", resumewait_setup);
> __setup("resumedelay=", resumedelay_setup);
> __setup("nohibernate", nohibernate_setup);
> __setup("kaslr", kaslr_nohibernate_setup);
> +__setup("page_poison=", page_poison_nohibernate_setup);
> diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
> index 25c98ae..3d3b954 100644
> --- a/mm/Kconfig.debug
> +++ b/mm/Kconfig.debug
> @@ -48,3 +48,17 @@ config PAGE_POISONING_NO_SANITY
>
> If you are only interested in sanitization, say Y. Otherwise
> say N.
> +
> +config PAGE_POISONING_ZERO
> + bool "Use zero for poisoning instead of random data"
> + depends on PAGE_POISONING
> + ---help---
> + Instead of using the existing poison value, fill the pages with
> + zeros. This makes it harder to detect when errors are occurring
> + due to sanitization but the zeroing at free means that it is
> + no longer necessary to write zeros when GFP_ZERO is used on
> + allocation.
May be worth noting the security benefit in this help text.
> +
> + Enabling page poisoning with this option will disable hibernation
This isn't obvious to me. It looks like you need to both use
CONFIG_PAGE_POISOING_ZERO and put "page_poison=on" on the command line
to enable it?
-Kees
> +
> + If unsure, say N
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index cc4762a..59bd9dc 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1382,15 +1382,24 @@ static inline int check_new_page(struct page *page)
> return 0;
> }
>
> +static inline bool free_pages_prezeroed(bool poisoned)
> +{
> + return IS_ENABLED(CONFIG_PAGE_POISONING_ZERO) &&
> + page_poisoning_enabled() && poisoned;
> +}
> +
> static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
> int alloc_flags)
> {
> int i;
> + bool poisoned = true;
>
> for (i = 0; i < (1 << order); i++) {
> struct page *p = page + i;
> if (unlikely(check_new_page(p)))
> return 1;
> + if (poisoned)
> + poisoned &= page_is_poisoned(p);
> }
>
> set_page_private(page, 0);
> @@ -1401,7 +1410,7 @@ static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
> kernel_poison_pages(page, 1 << order, 1);
> kasan_alloc_pages(page, order);
>
> - if (gfp_flags & __GFP_ZERO)
> + if (!free_pages_prezeroed(poisoned) && (gfp_flags & __GFP_ZERO))
> for (i = 0; i < (1 << order); i++)
> clear_highpage(page + i);
>
> diff --git a/mm/page_ext.c b/mm/page_ext.c
> index 292ca7b..2d864e6 100644
> --- a/mm/page_ext.c
> +++ b/mm/page_ext.c
> @@ -106,12 +106,15 @@ struct page_ext *lookup_page_ext(struct page *page)
> struct page_ext *base;
>
> base = NODE_DATA(page_to_nid(page))->node_page_ext;
> -#ifdef CONFIG_DEBUG_VM
> +#if defined(CONFIG_DEBUG_VM) || defined(CONFIG_PAGE_POISONING)
> /*
> * The sanity checks the page allocator does upon freeing a
> * page can reach here before the page_ext arrays are
> * allocated when feeding a range of pages to the allocator
> * for the first time during bootup or memory hotplug.
> + *
> + * This check is also necessary for ensuring page poisoning
> + * works as expected when enabled
> */
> if (unlikely(!base))
> return NULL;
> @@ -180,12 +183,15 @@ struct page_ext *lookup_page_ext(struct page *page)
> {
> unsigned long pfn = page_to_pfn(page);
> struct mem_section *section = __pfn_to_section(pfn);
> -#ifdef CONFIG_DEBUG_VM
> +#if defined(CONFIG_DEBUG_VM) || defined(CONFIG_PAGE_POISONING)
> /*
> * The sanity checks the page allocator does upon freeing a
> * page can reach here before the page_ext arrays are
> * allocated when feeding a range of pages to the allocator
> * for the first time during bootup or memory hotplug.
> + *
> + * This check is also necessary for ensuring page poisoning
> + * works as expected when enabled
> */
> if (!section->page_ext)
> return NULL;
> diff --git a/mm/page_poison.c b/mm/page_poison.c
> index 89d3bc7..479e7ea 100644
> --- a/mm/page_poison.c
> +++ b/mm/page_poison.c
> @@ -71,11 +71,14 @@ static inline void clear_page_poison(struct page *page)
> __clear_bit(PAGE_EXT_DEBUG_POISON, &page_ext->flags);
> }
>
> -static inline bool page_poison(struct page *page)
> +bool page_is_poisoned(struct page *page)
> {
> struct page_ext *page_ext;
>
> page_ext = lookup_page_ext(page);
> + if (!page_ext)
> + return false;
> +
> return test_bit(PAGE_EXT_DEBUG_POISON, &page_ext->flags);
> }
>
> @@ -137,7 +140,7 @@ static void unpoison_page(struct page *page)
> {
> void *addr;
>
> - if (!page_poison(page))
> + if (!page_is_poisoned(page))
> return;
>
> addr = kmap_atomic(page);
> --
> 2.5.0
>
--
Kees Cook
Chrome OS & Brillo Security
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