scudo: Interleave odd and even tags for adjacent blocks.

This guarantees that we will detect a buffer overflow or underflow
that overwrites an adjacent block. This spatial guarantee is similar
to the temporal guarantee that we provide for immediate use-after-free.

Enabling odd/even tags involves a tradeoff between use-after-free
detection and buffer overflow detection. Odd/even tags make it more
likely for buffer overflows to be detected by increasing the size of
the guaranteed "red zone" around the allocation, but on the other
hand use-after-free is less likely to be detected because the tag
space for any particular chunk is cut in half. Therefore we introduce
a tuning setting to control whether odd/even tags are enabled.

Differential Revision: https://reviews.llvm.org/D84361

compiler-rt/lib/scudo/standalone/combined.h

@@ -41,7 +41,7 @@ extern "C" size_t android_unsafe_frame_pointer_chase(scudo::uptr *buf,
 
 namespace scudo {
 
-enum class Option { ReleaseInterval };
+enum class Option { ReleaseInterval, MemtagTuning };
 
 template <class Params, void (*PostInitCallback)(void) = EmptyCallback>
 class Allocator {
@@ -154,6 +154,7 @@ class Allocator {
     Options.DeallocTypeMismatch = getFlags()->dealloc_type_mismatch;
     Options.DeleteSizeMismatch = getFlags()->delete_size_mismatch;
     Options.TrackAllocationStacks = false;
+    Options.UseOddEvenTags = true;
     Options.QuarantineMaxChunkSize =
         static_cast<u32>(getFlags()->quarantine_max_chunk_size);
 
@@ -251,6 +252,19 @@ class Allocator {
 #endif
   }
 
+  uptr computeOddEvenMaskForPointerMaybe(uptr Ptr, uptr Size) {
+    if (!Options.UseOddEvenTags)
+      return 0;
+
+    // If a chunk's tag is odd, we want the tags of the surrounding blocks to be
+    // even, and vice versa. Blocks are laid out Size bytes apart, and adding
+    // Size to Ptr will flip the least significant set bit of Size in Ptr, so
+    // that bit will have the pattern 010101... for consecutive blocks, which we
+    // can use to determine which tag mask to use.
+    return (Ptr & (1ULL << getLeastSignificantSetBitIndex(Size))) ? 0xaaaa
+                                                                  : 0x5555;
+  }
+
   NOINLINE void *allocate(uptr Size, Chunk::Origin Origin,
                           uptr Alignment = MinAlignment,
                           bool ZeroContents = false) {
@@ -350,7 +364,8 @@ class Allocator {
       if (UNLIKELY(useMemoryTagging())) {
         uptr PrevUserPtr;
         Chunk::UnpackedHeader Header;
-        const uptr BlockEnd = BlockUptr + PrimaryT::getSizeByClassId(ClassId);
+        const uptr BlockSize = PrimaryT::getSizeByClassId(ClassId);
+        const uptr BlockEnd = BlockUptr + BlockSize;
         // If possible, try to reuse the UAF tag that was set by deallocate().
         // For simplicity, only reuse tags if we have the same start address as
         // the previous allocation. This handles the majority of cases since
@@ -400,7 +415,9 @@ class Allocator {
             memset(TaggedPtr, 0, archMemoryTagGranuleSize());
           }
         } else {
-          TaggedPtr = prepareTaggedChunk(Ptr, Size, BlockEnd);
+          const uptr OddEvenMask =
+              computeOddEvenMaskForPointerMaybe(BlockUptr, BlockSize);
+          TaggedPtr = prepareTaggedChunk(Ptr, Size, OddEvenMask, BlockEnd);
         }
         storeAllocationStackMaybe(Ptr);
       } else if (UNLIKELY(FillContents != NoFill)) {
@@ -680,6 +697,23 @@ class Allocator {
       Secondary.setReleaseToOsIntervalMs(static_cast<s32>(Value));
       return true;
     }
+    if (O == Option::MemtagTuning) {
+      // Enabling odd/even tags involves a tradeoff between use-after-free
+      // detection and buffer overflow detection. Odd/even tags make it more
+      // likely for buffer overflows to be detected by increasing the size of
+      // the guaranteed "red zone" around the allocation, but on the other hand
+      // use-after-free is less likely to be detected because the tag space for
+      // any particular chunk is cut in half. Therefore we use this tuning
+      // setting to control whether odd/even tags are enabled.
+      if (Value == M_MEMTAG_TUNING_BUFFER_OVERFLOW) {
+        Options.UseOddEvenTags = true;
+        return true;
+      }
+      if (Value == M_MEMTAG_TUNING_UAF) {
+        Options.UseOddEvenTags = false;
+        return true;
+      }
+    }
     return false;
   }
 
@@ -921,6 +955,7 @@ class Allocator {
     u8 DeallocTypeMismatch : 1; // dealloc_type_mismatch
     u8 DeleteSizeMismatch : 1;  // delete_size_mismatch
     u8 TrackAllocationStacks : 1;
+    u8 UseOddEvenTags : 1;
     u32 QuarantineMaxChunkSize; // quarantine_max_chunk_size
   } Options;
 
@@ -987,9 +1022,13 @@ class Allocator {
     if (UNLIKELY(NewHeader.ClassId && useMemoryTagging())) {
       u8 PrevTag = extractTag(loadTag(reinterpret_cast<uptr>(Ptr)));
       uptr TaggedBegin, TaggedEnd;
+      const uptr OddEvenMask = computeOddEvenMaskForPointerMaybe(
+          reinterpret_cast<uptr>(getBlockBegin(Ptr, &NewHeader)),
+          SizeClassMap::getSizeByClassId(NewHeader.ClassId));
       // Exclude the previous tag so that immediate use after free is detected
       // 100% of the time.
-      setRandomTag(Ptr, Size, 1UL << PrevTag, &TaggedBegin, &TaggedEnd);
+      setRandomTag(Ptr, Size, OddEvenMask | (1UL << PrevTag), &TaggedBegin,
+                   &TaggedEnd);
       storeDeallocationStackMaybe(Ptr, PrevTag);
     }
     // If the quarantine is disabled, the actual size of a chunk is 0 or larger

compiler-rt/lib/scudo/standalone/include/scudo/interface.h

@@ -10,6 +10,7 @@
 #define SCUDO_INTERFACE_H_
 
 #include <stddef.h>
+#include <stdint.h>
 
 extern "C" {
 
@@ -105,6 +106,29 @@ size_t __scudo_get_stack_depot_size();
 const char *__scudo_get_region_info_addr();
 size_t __scudo_get_region_info_size();
 
+#ifndef M_DECAY_TIME
+#define M_DECAY_TIME -100
+#endif
+
+#ifndef M_PURGE
+#define M_PURGE -101
+#endif
+
+// Tune the allocator's choice of memory tags to make it more likely that
+// a certain class of memory errors will be detected. The value argument should
+// be one of the enumerators of the scudo_memtag_tuning enum below.
+#ifndef M_MEMTAG_TUNING
+#define M_MEMTAG_TUNING -102
+#endif
+
+enum scudo_memtag_tuning {
+  // Tune for buffer overflows.
+  M_MEMTAG_TUNING_BUFFER_OVERFLOW,
+
+  // Tune for use-after-free.
+  M_MEMTAG_TUNING_UAF,
+};
+
 } // extern "C"
 
 #endif // SCUDO_INTERFACE_H_

compiler-rt/lib/scudo/standalone/memtag.h

@@ -126,7 +126,8 @@ inline void setRandomTag(void *Ptr, uptr Size, uptr ExcludeMask,
       : "memory");
 }
 
-inline void *prepareTaggedChunk(void *Ptr, uptr Size, uptr BlockEnd) {
+inline void *prepareTaggedChunk(void *Ptr, uptr Size, uptr ExcludeMask,
+                                uptr BlockEnd) {
   // Prepare the granule before the chunk to store the chunk header by setting
   // its tag to 0. Normally its tag will already be 0, but in the case where a
   // chunk holding a low alignment allocation is reused for a higher alignment
@@ -138,7 +139,7 @@ inline void *prepareTaggedChunk(void *Ptr, uptr Size, uptr BlockEnd) {
                        : "memory");
 
   uptr TaggedBegin, TaggedEnd;
-  setRandomTag(Ptr, Size, 0, &TaggedBegin, &TaggedEnd);
+  setRandomTag(Ptr, Size, ExcludeMask, &TaggedBegin, &TaggedEnd);
 
   // Finally, set the tag of the granule past the end of the allocation to 0,
   // to catch linear overflows even if a previous larger allocation used the
@@ -235,9 +236,11 @@ inline void setRandomTag(void *Ptr, uptr Size, uptr ExcludeMask,
   UNREACHABLE("memory tagging not supported");
 }
 
-inline void *prepareTaggedChunk(void *Ptr, uptr Size, uptr BlockEnd) {
+inline void *prepareTaggedChunk(void *Ptr, uptr Size, uptr ExcludeMask,
+                                uptr BlockEnd) {
   (void)Ptr;
   (void)Size;
+  (void)ExcludeMask;
   (void)BlockEnd;
   UNREACHABLE("memory tagging not supported");
 }

compiler-rt/lib/scudo/standalone/tests/combined_test.cpp

@@ -469,3 +469,42 @@ TEST(ScudoCombinedTest, FullRegion) {
   }
   EXPECT_EQ(FailedAllocationsCount, 0U);
 }
+
+TEST(ScudoCombinedTest, OddEven) {
+  using AllocatorT = TestAllocator<scudo::AndroidConfig>;
+  using SizeClassMap = AllocatorT::PrimaryT::SizeClassMap;
+  auto Allocator = std::make_unique<AllocatorT>();
+
+  if (!Allocator->useMemoryTagging())
+    return;
+
+  auto CheckOddEven = [](scudo::uptr P1, scudo::uptr P2) {
+    scudo::uptr Tag1 = scudo::extractTag(scudo::loadTag(P1));
+    scudo::uptr Tag2 = scudo::extractTag(scudo::loadTag(P2));
+    EXPECT_NE(Tag1 % 2, Tag2 % 2);
+  };
+
+  for (scudo::uptr ClassId = 1U; ClassId <= SizeClassMap::LargestClassId;
+       ClassId++) {
+    const scudo::uptr Size = SizeClassMap::getSizeByClassId(ClassId);
+
+    std::set<scudo::uptr> Ptrs;
+    bool Found = false;
+    for (unsigned I = 0; I != 65536; ++I) {
+      scudo::uptr P = scudo::untagPointer(reinterpret_cast<scudo::uptr>(
+          Allocator->allocate(Size - scudo::Chunk::getHeaderSize(), Origin)));
+      if (Ptrs.count(P - Size)) {
+        Found = true;
+        CheckOddEven(P, P - Size);
+        break;
+      }
+      if (Ptrs.count(P + Size)) {
+        Found = true;
+        CheckOddEven(P, P + Size);
+        break;
+      }
+      Ptrs.insert(P);
+    }
+    EXPECT_TRUE(Found);
+  }
+}

compiler-rt/lib/scudo/standalone/tests/wrappers_c_test.cpp

@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "scudo/interface.h"
 #include "tests/scudo_unit_test.h"
 
 #include <errno.h>
@@ -188,14 +189,6 @@ TEST(ScudoWrappersCTest, Realloc) {
   }
 }
 
-#ifndef M_DECAY_TIME
-#define M_DECAY_TIME -100
-#endif
-
-#ifndef M_PURGE
-#define M_PURGE -101
-#endif
-
 #if !SCUDO_FUCHSIA
 TEST(ScudoWrappersCTest, MallOpt) {
   errno = 0;

compiler-rt/lib/scudo/standalone/wrappers_c.h

@@ -41,12 +41,4 @@ struct __scudo_mallinfo {
 #define SCUDO_MALLINFO __scudo_mallinfo
 #endif
 
-#ifndef M_DECAY_TIME
-#define M_DECAY_TIME -100
-#endif
-
-#ifndef M_PURGE
-#define M_PURGE -101
-#endif
-
 #endif // SCUDO_WRAPPERS_C_H_

compiler-rt/lib/scudo/standalone/wrappers_c.inc

@@ -173,6 +173,9 @@ INTERFACE WEAK int SCUDO_PREFIX(mallopt)(int param, UNUSED int value) {
   } else if (param == M_PURGE) {
     SCUDO_ALLOCATOR.releaseToOS();
     return 1;
+  } else if (param == M_MEMTAG_TUNING) {
+    return SCUDO_ALLOCATOR.setOption(scudo::Option::MemtagTuning,
+                                     static_cast<scudo::sptr>(value));
   }
   return 0;
 }