hwasan: Improve precision of checks using short granule tags.

A short granule is a granule of size between 1 and `TG-1` bytes. The size
of a short granule is stored at the location in shadow memory where the
granule's tag is normally stored, while the granule's actual tag is stored
in the last byte of the granule. This means that in order to verify that a
pointer tag matches a memory tag, HWASAN must check for two possibilities:

* the pointer tag is equal to the memory tag in shadow memory, or
* the shadow memory tag is actually a short granule size, the value being loaded
  is in bounds of the granule and the pointer tag is equal to the last byte of
  the granule.

Pointer tags between 1 to `TG-1` are possible and are as likely as any other
tag. This means that these tags in memory have two interpretations: the full
tag interpretation (where the pointer tag is between 1 and `TG-1` and the
last byte of the granule is ordinary data) and the short tag interpretation
(where the pointer tag is stored in the granule).

When HWASAN detects an error near a memory tag between 1 and `TG-1`, it
will show both the memory tag and the last byte of the granule. Currently,
it is up to the user to disambiguate the two possibilities.

Because this functionality obsoletes the right aligned heap feature of
the HWASAN memory allocator (and because we can no longer easily test
it), the feature is removed.

Also update the documentation to cover both short granule tags and
outlined checks.

Differential Revision: https://reviews.llvm.org/D63908

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@365551 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 27 insertions, 52 deletions

diff --git a/lib/hwasan/hwasan_allocator.cpp b/lib/hwasan/hwasan_allocator.cpp
index fd5248796..b4fae5820 100644
--- a/lib/hwasan/hwasan_allocator.cpp
+++ b/lib/hwasan/hwasan_allocator.cpp
@@ -16,6 +16,7 @@
 #include "sanitizer_common/sanitizer_stackdepot.h"
 #include "hwasan.h"
 #include "hwasan_allocator.h"
+#include "hwasan_checks.h"
 #include "hwasan_mapping.h"
 #include "hwasan_malloc_bisect.h"
 #include "hwasan_thread.h"
@@ -42,13 +43,8 @@ enum RightAlignMode {
   kRightAlignAlways
 };
 
-// These two variables are initialized from flags()->malloc_align_right
-// in HwasanAllocatorInit and are never changed afterwards.
-static RightAlignMode right_align_mode = kRightAlignNever;
-static bool right_align_8 = false;
-
 // Initialized in HwasanAllocatorInit, an never changed.
-static ALIGNED(16) u8 tail_magic[kShadowAlignment];
+static ALIGNED(16) u8 tail_magic[kShadowAlignment - 1];
 
 bool HwasanChunkView::IsAllocated() const {
   return metadata_ && metadata_->alloc_context_id && metadata_->requested_size;
@@ -58,8 +54,6 @@ bool HwasanChunkView::IsAllocated() const {
 static uptr AlignRight(uptr addr, uptr requested_size) {
   uptr tail_size = requested_size % kShadowAlignment;
   if (!tail_size) return addr;
-  if (right_align_8)
-    return tail_size > 8 ? addr : addr + 8;
   return addr + kShadowAlignment - tail_size;
 }
 
@@ -95,30 +89,7 @@ void HwasanAllocatorInit() {
                        !flags()->disable_allocator_tagging);
   SetAllocatorMayReturnNull(common_flags()->allocator_may_return_null);
   allocator.Init(common_flags()->allocator_release_to_os_interval_ms);
-  switch (flags()->malloc_align_right) {
-    case 0: break;
-    case 1:
-      right_align_mode = kRightAlignSometimes;
-      right_align_8 = false;
-      break;
-    case 2:
-      right_align_mode = kRightAlignAlways;
-      right_align_8 = false;
-      break;
-    case 8:
-      right_align_mode = kRightAlignSometimes;
-      right_align_8 = true;
-      break;
-    case 9:
-      right_align_mode = kRightAlignAlways;
-      right_align_8 = true;
-      break;
-    default:
-      Report("ERROR: unsupported value of malloc_align_right flag: %d\n",
-             flags()->malloc_align_right);
-      Die();
-  }
-  for (uptr i = 0; i < kShadowAlignment; i++)
+  for (uptr i = 0; i < sizeof(tail_magic); i++)
     tail_magic[i] = GetCurrentThread()->GenerateRandomTag();
 }
 
@@ -172,9 +143,10 @@ static void *HwasanAllocate(StackTrace *stack, uptr orig_size, uptr alignment,
     uptr fill_size = Min(size, (uptr)flags()->max_malloc_fill_size);
     internal_memset(allocated, flags()->malloc_fill_byte, fill_size);
   }
-  if (!right_align_mode)
+  if (size != orig_size) {
     internal_memcpy(reinterpret_cast<u8 *>(allocated) + orig_size, tail_magic,
-                    size - orig_size);
+                    size - orig_size - 1);
+  }
 
   void *user_ptr = allocated;
   // Tagging can only be skipped when both tag_in_malloc and tag_in_free are
@@ -182,19 +154,21 @@ static void *HwasanAllocate(StackTrace *stack, uptr orig_size, uptr alignment,
   // retag to 0.
   if ((flags()->tag_in_malloc || flags()->tag_in_free) &&
       atomic_load_relaxed(&hwasan_allocator_tagging_enabled)) {
-    tag_t tag = flags()->tag_in_malloc && malloc_bisect(stack, orig_size)
-                    ? (t ? t->GenerateRandomTag() : kFallbackAllocTag)
-                    : 0;
-    user_ptr = (void *)TagMemoryAligned((uptr)user_ptr, size, tag);
-  }
-
-  if ((orig_size % kShadowAlignment) && (alignment <= kShadowAlignment) &&
-      right_align_mode) {
-    uptr as_uptr = reinterpret_cast<uptr>(user_ptr);
-    if (right_align_mode == kRightAlignAlways ||
-        GetTagFromPointer(as_uptr) & 1) {  // use a tag bit as a random bit.
-      user_ptr = reinterpret_cast<void *>(AlignRight(as_uptr, orig_size));
-      meta->right_aligned = 1;
+    if (flags()->tag_in_malloc && malloc_bisect(stack, orig_size)) {
+      tag_t tag = t ? t->GenerateRandomTag() : kFallbackAllocTag;
+      uptr tag_size = orig_size ? orig_size : 1;
+      uptr full_granule_size = RoundDownTo(tag_size, kShadowAlignment);
+      user_ptr =
+          (void *)TagMemoryAligned((uptr)user_ptr, full_granule_size, tag);
+      if (full_granule_size != tag_size) {
+        u8 *short_granule =
+            reinterpret_cast<u8 *>(allocated) + full_granule_size;
+        TagMemoryAligned((uptr)short_granule, kShadowAlignment,
+                         tag_size % kShadowAlignment);
+        short_granule[kShadowAlignment - 1] = tag;
+      }
+    } else {
+      user_ptr = (void *)TagMemoryAligned((uptr)user_ptr, size, 0);
     }
   }
 
@@ -204,10 +178,10 @@ static void *HwasanAllocate(StackTrace *stack, uptr orig_size, uptr alignment,
 
 static bool PointerAndMemoryTagsMatch(void *tagged_ptr) {
   CHECK(tagged_ptr);
-  tag_t ptr_tag = GetTagFromPointer(reinterpret_cast<uptr>(tagged_ptr));
+  uptr tagged_uptr = reinterpret_cast<uptr>(tagged_ptr);
   tag_t mem_tag = *reinterpret_cast<tag_t *>(
       MemToShadow(reinterpret_cast<uptr>(UntagPtr(tagged_ptr))));
-  return ptr_tag == mem_tag;
+  return PossiblyShortTagMatches(mem_tag, tagged_uptr, 1);
 }
 
 static void HwasanDeallocate(StackTrace *stack, void *tagged_ptr) {
@@ -228,14 +202,15 @@ static void HwasanDeallocate(StackTrace *stack, void *tagged_ptr) {
 
   // Check tail magic.
   uptr tagged_size = TaggedSize(orig_size);
-  if (flags()->free_checks_tail_magic && !right_align_mode && orig_size) {
-    uptr tail_size = tagged_size - orig_size;
+  if (flags()->free_checks_tail_magic && orig_size &&
+      tagged_size != orig_size) {
+    uptr tail_size = tagged_size - orig_size - 1;
     CHECK_LT(tail_size, kShadowAlignment);
     void *tail_beg = reinterpret_cast<void *>(
         reinterpret_cast<uptr>(aligned_ptr) + orig_size);
     if (tail_size && internal_memcmp(tail_beg, tail_magic, tail_size))
       ReportTailOverwritten(stack, reinterpret_cast<uptr>(tagged_ptr),
-                            orig_size, tail_size, tail_magic);
+                            orig_size, tail_magic);
   }
 
   meta->requested_size = 0;