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

Greg Hackmann
2017-07-06T22:01:40Z

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