[PATCH v3 0/4] Sanitizing freed pages

[PATCH v3 0/4] Sanitizing freed pages

[Anisse Astier]

Hi,

I'm trying revive an old debate here[1], though with a simpler approach than
was previously tried. This patch series implements a new option to sanitize
freed pages, a (very) small subset of what is done in PaX/grsecurity[3],
inspired by a previous submission [4].

The first patch is fairly independent, and could be taken as-is. The second is
the meat and should be straight-forward to review.

There are a few different uses that this can cover:
 - some cases of use-after-free could be detected (crashes), although this not
   as efficient as KAsan/kmemcheck
 - it can help with long-term memory consumption in an environment with
   multiple VMs and Kernel Same-page Merging on the host. [2]
 - finally, it can reduce infoleaks, although this is hard to measure.

The approach is voluntarily kept as simple as possible. A single configuration
option, no command line option, no sysctl nob. It can of course be changed,
although I'd be wary of runtime-configuration options that could be used for
races.

I haven't been able to measure a meaningful performance difference when
compiling a (in-cache) kernel; I'd be interested to see what difference it
makes with your particular workload/hardware (I suspect mine is CPU-bound on
this small laptop).

Second patch fixes the hibernate use case which will load all the pages of the restored kernel, and then jump into it, leaving the loader kernel pages hanging around unclean. We use the free pages bitmap to know which pages should be cleaned after restore.

Fourth patch is debug code that can be used to find issues if this feature fails on your system. It shouldn't necessarily be merged.

Changes since v2:
 - reorder patches to fix hibernate first
 - update debug patch to use memchr_inv
 - cc linux-pm and maintainers

Changes since v1:
 - fix some issues raised by David Rientjes, Andi Kleen and PaX Team.
 - add hibernate fix (third patch)
 - add debug code, this is "just in case" someone has an issue with this
   feature. Not sure if it should be merged.


[1] https://lwn.net/Articles/334747/
[2] https://staff.aist.go.jp/k.suzaki/EuroSec12-SUZAKI-revised2.pdf
[3] http://en.wikibooks.org/wiki/Grsecurity/Appendix/Grsecurity_and_PaX_Configuration_Options#Sanitize_all_freed_memory
[4] http://article.gmane.org/gmane.linux.kernel.mm/34398



Anisse Astier (4):
  mm/page_alloc.c: cleanup obsolete KM_USER*
  mm/page_alloc.c: add config option to sanitize freed pages
  PM / Hibernate: fix SANITIZE_FREED_PAGES
  mm: Add debug code for SANITIZE_FREED_PAGES

 kernel/power/hibernate.c |  7 ++++++-
 kernel/power/power.h     |  4 ++++
 kernel/power/snapshot.c  | 24 ++++++++++++++++++++++
 mm/Kconfig               | 22 ++++++++++++++++++++
 mm/page_alloc.c          | 52 ++++++++++++++++++++++++++++++++++--------------
 5 files changed, 93 insertions(+), 16 deletions(-)

-- 
1.9.3

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[PATCH v3 1/4] mm/page_alloc.c: cleanup obsolete KM_USER*

[Anisse Astier]

It's been five years now that KM_* kmap flags have been removed and
that we can call clear_highpage from any context. So we remove
prep_zero_pages accordingly.

Signed-off-by: Anisse Astier <anisse@astier.eu>
---
 mm/page_alloc.c | 17 ++---------------
 1 file changed, 2 insertions(+), 15 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ebffa0e..4d5ce6e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -380,20 +380,6 @@ void prep_compound_page(struct page *page, unsigned long order)
 	}
 }
 
-static inline void prep_zero_page(struct page *page, unsigned int order,
-							gfp_t gfp_flags)
-{
-	int i;
-
-	/*
-	 * clear_highpage() will use KM_USER0, so it's a bug to use __GFP_ZERO
-	 * and __GFP_HIGHMEM from hard or soft interrupt context.
-	 */
-	VM_BUG_ON((gfp_flags & __GFP_HIGHMEM) && in_interrupt());
-	for (i = 0; i < (1 << order); i++)
-		clear_highpage(page + i);
-}
-
 #ifdef CONFIG_DEBUG_PAGEALLOC
 unsigned int _debug_guardpage_minorder;
 bool _debug_pagealloc_enabled __read_mostly;
@@ -975,7 +961,8 @@ static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
 	kasan_alloc_pages(page, order);
 
 	if (gfp_flags & __GFP_ZERO)
-		prep_zero_page(page, order, gfp_flags);
+		for (i = 0; i < (1 << order); i++)
+			clear_highpage(page + i);
 
 	if (order && (gfp_flags & __GFP_COMP))
 		prep_compound_page(page, order);
-- 
1.9.3

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[PATCH v3 2/4] PM / Hibernate: prepare for SANITIZE_FREED_PAGES

[Anisse Astier]

SANITIZE_FREED_PAGES feature relies on having all pages going through
the free_pages_prepare path in order to be cleared before being used. In
the hibernate use case, pages will automagically appear in the system
without being cleared.

This patch will make sure free pages are cleared on resume; when we'll
enable SANITIZE_FREED_PAGES. We free the pages just after resume because
we can't do it later: going through any device resume code might
allocate some memory and invalidate the free pages bitmap.

Signed-off-by: Anisse Astier <anisse@astier.eu>
---
 kernel/power/hibernate.c |  7 ++++++-
 kernel/power/power.h     |  4 ++++
 kernel/power/snapshot.c  | 21 +++++++++++++++++++++
 3 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/kernel/power/hibernate.c b/kernel/power/hibernate.c
index 2329daa..3193b9a 100644
--- a/kernel/power/hibernate.c
+++ b/kernel/power/hibernate.c
@@ -305,9 +305,14 @@ static int create_image(int platform_mode)
 			error);
 	/* Restore control flow magically appears here */
 	restore_processor_state();
-	if (!in_suspend)
+	if (!in_suspend) {
 		events_check_enabled = false;
 
+#ifdef CONFIG_SANITIZE_FREED_PAGES
+		clear_free_pages();
+		printk(KERN_INFO "PM: free pages cleared after restore\n");
+#endif
+	}
 	platform_leave(platform_mode);
 
  Power_up:
diff --git a/kernel/power/power.h b/kernel/power/power.h
index ce9b832..26b2101 100644
--- a/kernel/power/power.h
+++ b/kernel/power/power.h
@@ -92,6 +92,10 @@ extern int create_basic_memory_bitmaps(void);
 extern void free_basic_memory_bitmaps(void);
 extern int hibernate_preallocate_memory(void);
 
+#ifdef CONFIG_SANITIZE_FREED_PAGES
+extern void clear_free_pages(void);
+#endif
+
 /**
  *	Auxiliary structure used for reading the snapshot image data and
  *	metadata from and writing them to the list of page backup entries
diff --git a/kernel/power/snapshot.c b/kernel/power/snapshot.c
index 5235dd4..673ade1 100644
--- a/kernel/power/snapshot.c
+++ b/kernel/power/snapshot.c
@@ -1032,6 +1032,27 @@ void free_basic_memory_bitmaps(void)
 	pr_debug("PM: Basic memory bitmaps freed\n");
 }
 
+#ifdef CONFIG_SANITIZE_FREED_PAGES
+void clear_free_pages(void)
+{
+	struct memory_bitmap *bm = free_pages_map;
+	unsigned long pfn;
+
+	if (WARN_ON(!(free_pages_map)))
+		return;
+
+	memory_bm_position_reset(bm);
+	pfn = memory_bm_next_pfn(bm);
+	while (pfn != BM_END_OF_MAP) {
+		if (pfn_valid(pfn))
+			clear_highpage(pfn_to_page(pfn));
+
+		pfn = memory_bm_next_pfn(bm);
+	}
+	memory_bm_position_reset(bm);
+}
+#endif /* SANITIZE_FREED_PAGES */
+
 /**
  *	snapshot_additional_pages - estimate the number of additional pages
  *	be needed for setting up the suspend image data structures for given
-- 
1.9.3

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Re: [PATCH v3 2/4] PM / Hibernate: prepare for SANITIZE_FREED_PAGES

[Pavel Machek]

Hi!

> SANITIZE_FREED_PAGES feature relies on having all pages going through
> the free_pages_prepare path in order to be cleared before being used. In
> the hibernate use case, pages will automagically appear in the system
> without being cleared.
> 
> This patch will make sure free pages are cleared on resume; when we'll
> enable SANITIZE_FREED_PAGES. We free the pages just after resume because
> we can't do it later: going through any device resume code might
> allocate some memory and invalidate the free pages bitmap.
> 
> Signed-off-by: Anisse Astier <anisse@astier.eu>
> ---
>  kernel/power/hibernate.c |  7 ++++++-
>  kernel/power/power.h     |  4 ++++
>  kernel/power/snapshot.c  | 21 +++++++++++++++++++++
>  3 files changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/power/hibernate.c b/kernel/power/hibernate.c
> index 2329daa..3193b9a 100644
> --- a/kernel/power/hibernate.c
> +++ b/kernel/power/hibernate.c
> @@ -305,9 +305,14 @@ static int create_image(int platform_mode)
>  			error);
>  	/* Restore control flow magically appears here */
>  	restore_processor_state();
> -	if (!in_suspend)
> +	if (!in_suspend) {
>  		events_check_enabled = false;
>  
> +#ifdef CONFIG_SANITIZE_FREED_PAGES
> +		clear_free_pages();
> +		printk(KERN_INFO "PM: free pages cleared after restore\n");
> +#endif
> +	}
>  	platform_leave(platform_mode);
>  
>   Power_up:

Can you move the ifdef and the printk into the clear_free_pages?

This is not performance critical in any way...

Otherwise it looks good to me... if the sanitization is considered
useful. Did it catch some bugs in the past?

Thanks,
									Pavel
-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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Re: [PATCH v3 2/4] PM / Hibernate: prepare for SANITIZE_FREED_PAGES

[Anisse Astier]

Hi Pavel,

Thanks a lot for taking the time to review this.

On Sat, May 9, 2015 at 5:44 PM, Pavel Machek <pavel@ucw.cz> wrote:
>> +#ifdef CONFIG_SANITIZE_FREED_PAGES
>> +             clear_free_pages();
>> +             printk(KERN_INFO "PM: free pages cleared after restore\n");
>> +#endif
>> +     }
>>       platform_leave(platform_mode);
>>
>>   Power_up:
>
> Can you move the ifdef and the printk into the clear_free_pages?

Sure. I put the printk out originally because i thought there might be
other uses, but since this is the sole call site right now it
shouldn't be an issue.

>
> This is not performance critical in any way...
>
> Otherwise it looks good to me... if the sanitization is considered
> useful. Did it catch some bugs in the past?
>

I've read somewhere that users of grsecurity claim that it caught bugs
in some drivers, but I haven't verified that personally; it's probably
much less useful than kasan (or even the original grsec feature) as a
bug-catcher since it doesn't clear freed slab buffers.

I'll wait a few more days for more reviews before sending the next
version, particularly on the power management part, and in general on
the usefulness of such feature.

Regards,

Anisse

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Re: [PATCH v3 2/4] PM / Hibernate: prepare for SANITIZE_FREED_PAGES

[PaX Team]

On 11 May 2015 at 9:59, Anisse Astier wrote:

> > Otherwise it looks good to me... if the sanitization is considered
> > useful. Did it catch some bugs in the past?
> >
> 
> I've read somewhere that users of grsecurity claim that it caught bugs
> in some drivers, but I haven't verified that personally; it's probably
> much less useful than kasan (or even the original grsec feature) as a
> bug-catcher since it doesn't clear freed slab buffers.

the PaX SANITIZE feature wasn't developed for catching use-after-free bugs
but to help reduce data lifetime from the kernel while not killing too much
performance (this is why i was reluctant to add a finer grained version to
do slab object sanitization until Mathias Krause came up with a workable
compromise).

another reason page zeroing isn't good at catching these bugs is that the
0 fill value will produce NULL pointers which are often explicitly handled
already. on the other hand changing the fill value would not allow the
__GFP_ZERO performance optimization (the slab sanitization feature is a
different story however, we have a non-0 fill value and it keeps triggering
use-after-free bugs).

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[PATCH v3 3/4] mm/page_alloc.c: add config option to sanitize freed pages

[Anisse Astier]

This new config option will sanitize all freed pages. This is a pretty
low-level change useful to track some cases of use-after-free, help
kernel same-page merging in VM environments, and counter a few info
leaks.

Signed-off-by: Anisse Astier <anisse@astier.eu>
---
 mm/Kconfig      | 12 ++++++++++++
 mm/page_alloc.c | 12 ++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/mm/Kconfig b/mm/Kconfig
index 390214d..e9fb3bd 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -635,3 +635,15 @@ config MAX_STACK_SIZE_MB
 	  changed to a smaller value in which case that is used.
 
 	  A sane initial value is 80 MB.
+
+config SANITIZE_FREED_PAGES
+	bool "Sanitize memory pages after free"
+	default n
+	help
+	  This option is used to make sure all pages freed are zeroed. This is
+	  quite low-level and doesn't handle your slab buffers.
+	  It has various applications, from preventing some info leaks to
+	  helping kernel same-page merging in virtualised environments.
+	  Depending on your workload it will greatly reduce performance.
+
+	  If unsure, say N.
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4d5ce6e..c29e3a0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -795,6 +795,12 @@ static bool free_pages_prepare(struct page *page, unsigned int order)
 		debug_check_no_obj_freed(page_address(page),
 					   PAGE_SIZE << order);
 	}
+
+#ifdef CONFIG_SANITIZE_FREED_PAGES
+	for (i = 0; i < (1 << order); i++)
+		clear_highpage(page + i);
+#endif
+
 	arch_free_page(page, order);
 	kernel_map_pages(page, 1 << order, 0);
 
@@ -960,9 +966,15 @@ static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
 	kernel_map_pages(page, 1 << order, 1);
 	kasan_alloc_pages(page, order);
 
+#ifndef CONFIG_SANITIZE_FREED_PAGES
+	/* SANITIZE_FREED_PAGES relies implicitly on the fact that pages are
+	 * cleared before use, so we don't need gfp zero in the default case
+	 * because all pages go through the free_pages_prepare code path when
+	 * switching from bootmem to the default allocator */
 	if (gfp_flags & __GFP_ZERO)
 		for (i = 0; i < (1 << order); i++)
 			clear_highpage(page + i);
+#endif
 
 	if (order && (gfp_flags & __GFP_COMP))
 		prep_compound_page(page, order);
-- 
1.9.3

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[PATCH v3 4/4] mm: Add debug code for SANITIZE_FREED_PAGES

[Anisse Astier]

Add debug code for sanitize freed pages to print status and verify pages
at alloc to make sure they're clean. It can be useful if you have
crashes when using SANITIZE_FREED_PAGES.

Signed-off-by: Anisse Astier <anisse@astier.eu>
---
 kernel/power/snapshot.c |  8 ++++++--
 mm/Kconfig              | 10 ++++++++++
 mm/page_alloc.c         | 18 ++++++++++++++++++
 3 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/kernel/power/snapshot.c b/kernel/power/snapshot.c
index 673ade1..dfbfb5f 100644
--- a/kernel/power/snapshot.c
+++ b/kernel/power/snapshot.c
@@ -1044,9 +1044,13 @@ void clear_free_pages(void)
 	memory_bm_position_reset(bm);
 	pfn = memory_bm_next_pfn(bm);
 	while (pfn != BM_END_OF_MAP) {
-		if (pfn_valid(pfn))
+		if (pfn_valid(pfn)) {
+#ifdef CONFIG_SANITIZE_FREED_PAGES_DEBUG
+			printk(KERN_INFO "Clearing page %p\n",
+					page_address(pfn_to_page(pfn)));
+#endif
 			clear_highpage(pfn_to_page(pfn));
-
+		}
 		pfn = memory_bm_next_pfn(bm);
 	}
 	memory_bm_position_reset(bm);
diff --git a/mm/Kconfig b/mm/Kconfig
index e9fb3bd..95364f2 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -647,3 +647,13 @@ config SANITIZE_FREED_PAGES
 	  Depending on your workload it will greatly reduce performance.
 
 	  If unsure, say N.
+
+config SANITIZE_FREED_PAGES_DEBUG
+	bool "Debug sanitize pages feature"
+	default n
+	depends on SANITIZE_FREED_PAGES && DEBUG_KERNEL
+	help
+	  This option adds some debugging code for the SANITIZE_FREED_PAGES
+	  option, as well as verification code to ensure pages are really
+	  zeroed. Don't enable unless you want to debug this feature.
+	  If unsure, say N.
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c29e3a0..f6105e5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -975,6 +975,24 @@ static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
 		for (i = 0; i < (1 << order); i++)
 			clear_highpage(page + i);
 #endif
+#ifdef CONFIG_SANITIZE_FREED_PAGES_DEBUG
+	for (i = 0; i < (1 << order); i++) {
+		struct page *p = page + i;
+		void *kaddr = kmap_atomic(p);
+		void *found = memchr_inv(kaddr, 0, PAGE_SIZE);
+		kunmap_atomic(kaddr);
+
+		if (found) {
+			pr_err("page %p is not zero on alloc! %s\n",
+					page_address(p), (gfp_flags &
+						__GFP_ZERO) ?
+					"fixing." : "");
+			if (gfp_flags & __GFP_ZERO) {
+				clear_highpage(p);
+			}
+		}
+	}
+#endif
 
 	if (order && (gfp_flags & __GFP_COMP))
 		prep_compound_page(page, order);
-- 
1.9.3

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