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[PATCH 1/4] kasan: support alloca() poisoning

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  [PATCH 1/4] kasan: support alloca() poisoning Greg Hackmann <ghackmann@google.com> - 2017-07-07 00:10 +0200
    Re: [PATCH 1/4] kasan: support alloca() poisoning Greg Hackmann <ghackmann@google.com> - 2017-07-07 02:10 +0200
    Re: [PATCH 1/4] kasan: support alloca() poisoning Dmitry Vyukov <dvyukov@google.com> - 2017-07-10 10:50 +0200
      Re: [PATCH 1/4] kasan: support alloca() poisoning Greg Hackmann <ghackmann@google.com> - 2017-07-14 00:50 +0200
        Re: [PATCH 1/4] kasan: support alloca() poisoning Dmitry Vyukov <dvyukov@google.com> - 2017-07-14 08:20 +0200
    Re: [PATCH 1/4] kasan: support alloca() poisoning Greg Hackmann <ghackmann@google.com> - 2017-07-14 01:00 +0200

#1682746 — [PATCH 1/4] kasan: support alloca() poisoning

clang's AddressSanitizer implementation adds redzones on either side of
alloca()ed buffers.  These redzones are 32-byte aligned and at least 32
bytes long.

__asan_alloca_poison() is passed the size and address of the allocated
buffer, *excluding* the redzones on either side.  The left redzone will
always be to the immediate left of this buffer; but AddressSanitizer may
need to add padding between the end of the buffer and the right redzone.
If there are any 8-byte chunks inside this padding, we should poison
those too.

__asan_allocas_unpoison() is just passed the top and bottom of the
dynamic stack area, so unpoisoning is simpler.

Signed-off-by: Greg Hackmann <ghackmann@google.com>
---
 lib/test_kasan.c  | 22 ++++++++++++++++++++++
 mm/kasan/kasan.c  | 26 ++++++++++++++++++++++++++
 mm/kasan/kasan.h  |  8 ++++++++
 mm/kasan/report.c |  3 +++
 4 files changed, 59 insertions(+)

diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index a25c9763fce1..f774fcafb696 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c
@@ -473,6 +473,26 @@ static noinline void __init use_after_scope_test(void)
 	p[1023] = 1;
 }
 
+static noinline void __init kasan_alloca_oob_left(void)
+{
+	volatile int i = 10;
+	char alloca_array[i];
+	char *p = alloca_array - 1;
+
+	pr_info("out-of-bounds to left on alloca\n");
+	*(volatile char *)p;
+}
+
+static noinline void __init kasan_alloca_oob_right(void)
+{
+	volatile int i = 10;
+	char alloca_array[i];
+	char *p = alloca_array + round_up(i, 8);
+
+	pr_info("out-of-bounds to right on alloca\n");
+	*(volatile char *)p;
+}
+
 static int __init kmalloc_tests_init(void)
 {
 	/*
@@ -503,6 +523,8 @@ static int __init kmalloc_tests_init(void)
 	memcg_accounted_kmem_cache();
 	kasan_stack_oob();
 	kasan_global_oob();
+	kasan_alloca_oob_left();
+	kasan_alloca_oob_right();
 	ksize_unpoisons_memory();
 	copy_user_test();
 	use_after_scope_test();
diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
index c81549d5c833..892b626f564b 100644
--- a/mm/kasan/kasan.c
+++ b/mm/kasan/kasan.c
@@ -802,6 +802,32 @@ void __asan_unpoison_stack_memory(const void *addr, size_t size)
 }
 EXPORT_SYMBOL(__asan_unpoison_stack_memory);
 
+/* Emitted by compiler to poison alloca()ed objects. */
+void __asan_alloca_poison(unsigned long addr, size_t size)
+{
+	size_t rounded_up_size = round_up(size, KASAN_SHADOW_SCALE_SIZE);
+	size_t padding_size = round_up(size, KASAN_ALLOCA_REDZONE_SIZE) -
+			round_up(size, KASAN_SHADOW_SCALE_SIZE);
+
+	const void *left_redzone = (const void *)(addr -
+			KASAN_ALLOCA_REDZONE_SIZE);
+	const void *right_redzone = (const void *)(addr + rounded_up_size);
+
+	kasan_poison_shadow(left_redzone, KASAN_ALLOCA_REDZONE_SIZE,
+			KASAN_ALLOCA_LEFT);
+	kasan_poison_shadow(right_redzone,
+			padding_size + KASAN_ALLOCA_REDZONE_SIZE,
+			KASAN_ALLOCA_RIGHT);
+}
+EXPORT_SYMBOL(__asan_alloca_poison);
+
+/* Emitted by compiler to unpoison alloca()ed areas when the stack unwinds. */
+void __asan_allocas_unpoison(const void *stack_top, const void *stack_bottom)
+{
+	kasan_unpoison_shadow(stack_top, stack_bottom - stack_top);
+}
+EXPORT_SYMBOL(__asan_allocas_unpoison);
+
 #ifdef CONFIG_MEMORY_HOTPLUG
 static int kasan_mem_notifier(struct notifier_block *nb,
 			unsigned long action, void *data)
diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 1229298cce64..b857dc70d6a2 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -23,6 +23,14 @@
 #define KASAN_STACK_PARTIAL     0xF4
 #define KASAN_USE_AFTER_SCOPE   0xF8
 
+/*
+ * alloca redzone shadow values
+ */
+#define KASAN_ALLOCA_LEFT	0xCA
+#define KASAN_ALLOCA_RIGHT	0xCB
+
+#define KASAN_ALLOCA_REDZONE_SIZE	32
+
 /* Don't break randconfig/all*config builds */
 #ifndef KASAN_ABI_VERSION
 #define KASAN_ABI_VERSION 1
diff --git a/mm/kasan/report.c b/mm/kasan/report.c
index beee0e980e2d..c6a5b7ab9e3a 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -101,6 +101,9 @@ static const char *get_shadow_bug_type(struct kasan_access_info *info)
 		break;
 	case KASAN_USE_AFTER_SCOPE:
 		bug_type = "use-after-scope";
+	case KASAN_ALLOCA_LEFT:
+	case KASAN_ALLOCA_RIGHT:
+		bug_type = "alloca-out-of-bounds";
 		break;
 	}
 
-- 
2.13.2.725.g09c95d1e9-goog

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#1682823

On 07/06/2017 03:01 PM, Greg Hackmann wrote:
> @@ -101,6 +101,9 @@ static const char *get_shadow_bug_type(struct kasan_access_info *info)
>   		break;
>   	case KASAN_USE_AFTER_SCOPE:
>   		bug_type = "use-after-scope";
> +	case KASAN_ALLOCA_LEFT:
> +	case KASAN_ALLOCA_RIGHT:
> +		bug_type = "alloca-out-of-bounds";
>   		break;
>   	}

There needs to be a "break" above the new case statements.  I'll wait to 
see if there's any other feedback, then send out a V2 patch that fixes this.

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#1684059

On Fri, Jul 7, 2017 at 12:01 AM, Greg Hackmann <ghackmann@google.com> wrote:
> clang's AddressSanitizer implementation adds redzones on either side of
> alloca()ed buffers.  These redzones are 32-byte aligned and at least 32
> bytes long.
>
> __asan_alloca_poison() is passed the size and address of the allocated
> buffer, *excluding* the redzones on either side.  The left redzone will
> always be to the immediate left of this buffer; but AddressSanitizer may
> need to add padding between the end of the buffer and the right redzone.
> If there are any 8-byte chunks inside this padding, we should poison
> those too.
>
> __asan_allocas_unpoison() is just passed the top and bottom of the
> dynamic stack area, so unpoisoning is simpler.
>
> Signed-off-by: Greg Hackmann <ghackmann@google.com>
> ---
>  lib/test_kasan.c  | 22 ++++++++++++++++++++++
>  mm/kasan/kasan.c  | 26 ++++++++++++++++++++++++++
>  mm/kasan/kasan.h  |  8 ++++++++
>  mm/kasan/report.c |  3 +++
>  4 files changed, 59 insertions(+)
>
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index a25c9763fce1..f774fcafb696 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -473,6 +473,26 @@ static noinline void __init use_after_scope_test(void)
>         p[1023] = 1;
>  }
>
> +static noinline void __init kasan_alloca_oob_left(void)
> +{
> +       volatile int i = 10;
> +       char alloca_array[i];
> +       char *p = alloca_array - 1;
> +
> +       pr_info("out-of-bounds to left on alloca\n");
> +       *(volatile char *)p;
> +}
> +
> +static noinline void __init kasan_alloca_oob_right(void)
> +{
> +       volatile int i = 10;
> +       char alloca_array[i];
> +       char *p = alloca_array + round_up(i, 8);
> +
> +       pr_info("out-of-bounds to right on alloca\n");
> +       *(volatile char *)p;
> +}
> +
>  static int __init kmalloc_tests_init(void)
>  {
>         /*
> @@ -503,6 +523,8 @@ static int __init kmalloc_tests_init(void)
>         memcg_accounted_kmem_cache();
>         kasan_stack_oob();
>         kasan_global_oob();
> +       kasan_alloca_oob_left();
> +       kasan_alloca_oob_right();
>         ksize_unpoisons_memory();
>         copy_user_test();
>         use_after_scope_test();
> diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
> index c81549d5c833..892b626f564b 100644
> --- a/mm/kasan/kasan.c
> +++ b/mm/kasan/kasan.c
> @@ -802,6 +802,32 @@ void __asan_unpoison_stack_memory(const void *addr, size_t size)
>  }
>  EXPORT_SYMBOL(__asan_unpoison_stack_memory);
>
> +/* Emitted by compiler to poison alloca()ed objects. */
> +void __asan_alloca_poison(unsigned long addr, size_t size)
> +{
> +       size_t rounded_up_size = round_up(size, KASAN_SHADOW_SCALE_SIZE);
> +       size_t padding_size = round_up(size, KASAN_ALLOCA_REDZONE_SIZE) -
> +                       round_up(size, KASAN_SHADOW_SCALE_SIZE);

Perhaps s/round_up(size, KASAN_SHADOW_SCALE_SIZE)/rounded_up_size/
because we already calculated that.

> +
> +       const void *left_redzone = (const void *)(addr -
> +                       KASAN_ALLOCA_REDZONE_SIZE);
> +       const void *right_redzone = (const void *)(addr + rounded_up_size);

Please check that size is rounded to KASAN_ALLOCA_REDZONE_SIZE. That's
the expectation, right? That can change is clang silently.

> +       kasan_poison_shadow(left_redzone, KASAN_ALLOCA_REDZONE_SIZE,
> +                       KASAN_ALLOCA_LEFT);
> +       kasan_poison_shadow(right_redzone,
> +                       padding_size + KASAN_ALLOCA_REDZONE_SIZE,
> +                       KASAN_ALLOCA_RIGHT);

We also need to poison the unaligned part at the end of the object
from size to rounded_up_size. You can see how we do it for heap
objects.

> +}
> +EXPORT_SYMBOL(__asan_alloca_poison);
> +/* Emitted by compiler to unpoison alloca()ed areas when the stack unwinds. */
> +void __asan_allocas_unpoison(const void *stack_top, const void *stack_bottom)
> +{
> +       kasan_unpoison_shadow(stack_top, stack_bottom - stack_top);
> +}
> +EXPORT_SYMBOL(__asan_allocas_unpoison);
> +
>  #ifdef CONFIG_MEMORY_HOTPLUG
>  static int kasan_mem_notifier(struct notifier_block *nb,
>                         unsigned long action, void *data)
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index 1229298cce64..b857dc70d6a2 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -23,6 +23,14 @@
>  #define KASAN_STACK_PARTIAL     0xF4
>  #define KASAN_USE_AFTER_SCOPE   0xF8
>
> +/*
> + * alloca redzone shadow values
> + */
> +#define KASAN_ALLOCA_LEFT      0xCA
> +#define KASAN_ALLOCA_RIGHT     0xCB
> +
> +#define KASAN_ALLOCA_REDZONE_SIZE      32
> +
>  /* Don't break randconfig/all*config builds */
>  #ifndef KASAN_ABI_VERSION
>  #define KASAN_ABI_VERSION 1
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index beee0e980e2d..c6a5b7ab9e3a 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -101,6 +101,9 @@ static const char *get_shadow_bug_type(struct kasan_access_info *info)
>                 break;
>         case KASAN_USE_AFTER_SCOPE:
>                 bug_type = "use-after-scope";
> +       case KASAN_ALLOCA_LEFT:
> +       case KASAN_ALLOCA_RIGHT:
> +               bug_type = "alloca-out-of-bounds";
>                 break;
>         }
>
> --
> 2.13.2.725.g09c95d1e9-goog
>

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#1686978

Hi,

Thanks for taking a look at this patchstack.  I apologize for the delay 
in responding.

On 07/10/2017 01:44 AM, Dmitry Vyukov wrote:
>> +
>> +       const void *left_redzone = (const void *)(addr -
>> +                       KASAN_ALLOCA_REDZONE_SIZE);
>> +       const void *right_redzone = (const void *)(addr + rounded_up_size);
> 
> Please check that size is rounded to KASAN_ALLOCA_REDZONE_SIZE. That's
> the expectation, right? That can change is clang silently.
> 
>> +       kasan_poison_shadow(left_redzone, KASAN_ALLOCA_REDZONE_SIZE,
>> +                       KASAN_ALLOCA_LEFT);
>> +       kasan_poison_shadow(right_redzone,
>> +                       padding_size + KASAN_ALLOCA_REDZONE_SIZE,
>> +                       KASAN_ALLOCA_RIGHT);
> 
> We also need to poison the unaligned part at the end of the object
> from size to rounded_up_size. You can see how we do it for heap
> objects.

The expectation is that `size' is the exact size of the alloca()ed 
object.  `rounded_up_size' then adds the 0-7 bytes needed to adjust the 
size to the ASAN shadow scale.  So `addr + rounded_up_size' should be 
the correct place to start poisoning.

In retrospect this part of the code was pretty confusing.  How about 
this?  I think its intent is clearer, plus it's a closer match for the 
description in my commit message:

	unsigned long left_redzone_start;
	unsigned long object_end;
	unsigned long right_redzone_start, right_redzone_end;

	left_redzone_start = addr - KASAN_ALLOCA_REDZONE_SIZE;
	kasan_poison_shadow((const void *)left_redzone_start,
			KASAN_ALLOCA_REDZONE_SIZE,
			KASAN_ALLOCA_LEFT);

	object_end = round_up(addr + size, KASAN_SHADOW_SCALE_SIZE);
	right_redzone_start = round_up(object_end, KASAN_ALLOCA_REDZONE_SIZE);
	right_redzone_end = right_redzone_start + KASAN_ALLOCA_REDZONE_SIZE;
	kasan_poison_shadow((const void *)object_end,
			right_redzone_end - object_end,
			KASAN_ALLOCA_RIGHT);

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#1687088

On Fri, Jul 14, 2017 at 12:40 AM, Greg Hackmann <ghackmann@google.com> wrote:
> Hi,
>
> Thanks for taking a look at this patchstack.  I apologize for the delay in
> responding.
>
> On 07/10/2017 01:44 AM, Dmitry Vyukov wrote:
>>>
>>> +
>>> +       const void *left_redzone = (const void *)(addr -
>>> +                       KASAN_ALLOCA_REDZONE_SIZE);
>>> +       const void *right_redzone = (const void *)(addr +
>>> rounded_up_size);
>>
>>
>> Please check that size is rounded to KASAN_ALLOCA_REDZONE_SIZE. That's
>> the expectation, right? That can change is clang silently.
>>
>>> +       kasan_poison_shadow(left_redzone, KASAN_ALLOCA_REDZONE_SIZE,
>>> +                       KASAN_ALLOCA_LEFT);
>>> +       kasan_poison_shadow(right_redzone,
>>> +                       padding_size + KASAN_ALLOCA_REDZONE_SIZE,
>>> +                       KASAN_ALLOCA_RIGHT);
>>
>>
>> We also need to poison the unaligned part at the end of the object
>> from size to rounded_up_size. You can see how we do it for heap
>> objects.
>
>
> The expectation is that `size' is the exact size of the alloca()ed object.
> `rounded_up_size' then adds the 0-7 bytes needed to adjust the size to the
> ASAN shadow scale.  So `addr + rounded_up_size' should be the correct place
> to start poisoning.


We need to start poisoning at addr+size exactly.
Asan shadow scheme supports this. It's not possible to poison
beginning of an aligned 8-byte block, but leave tail unpoisoned. But
it is possible to poison tail of an aligned 8-byte block and leave
beginning unpoisoned. Look at what we do for kmalloc.


> In retrospect this part of the code was pretty confusing.  How about this?
> I think its intent is clearer, plus it's a closer match for the description
> in my commit message:
>
>         unsigned long left_redzone_start;
>         unsigned long object_end;
>         unsigned long right_redzone_start, right_redzone_end;
>
>         left_redzone_start = addr - KASAN_ALLOCA_REDZONE_SIZE;
>         kasan_poison_shadow((const void *)left_redzone_start,
>                         KASAN_ALLOCA_REDZONE_SIZE,
>                         KASAN_ALLOCA_LEFT);
>
>         object_end = round_up(addr + size, KASAN_SHADOW_SCALE_SIZE);
>         right_redzone_start = round_up(object_end,
> KASAN_ALLOCA_REDZONE_SIZE);
>         right_redzone_end = right_redzone_start + KASAN_ALLOCA_REDZONE_SIZE;
>         kasan_poison_shadow((const void *)object_end,
>                         right_redzone_end - object_end,
>                         KASAN_ALLOCA_RIGHT);

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#1686979

On 07/10/2017 03:30 AM, Andrey Ryabinin wrote:
> gcc now supports this too. So I think this patch should enable it.
> It's off by default so you'll have to add --param asan-instrument-allocas=1 into cflags
> to make it work

Thanks, will fix.  For now, it looks like I'll need to build gcc from 
git to test this?

>>   lib/test_kasan.c  | 22 ++++++++++++++++++++++
> 
> Tests would be better as a separate patch.

I was following the precedent in 828347f8f9a5 ("kasan: support 
use-after-scope detection") which added both at the same time.  But I 
can split the test off into a separate patch if you feel really strongly 
about it.

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