diff --git a/compiler-rt/lib/hwasan/hwasan_allocator.h b/compiler-rt/lib/hwasan/hwasan_allocator.h
--- a/compiler-rt/lib/hwasan/hwasan_allocator.h
+++ b/compiler-rt/lib/hwasan/hwasan_allocator.h
@@ -13,13 +13,15 @@
 #ifndef HWASAN_ALLOCATOR_H
 #define HWASAN_ALLOCATOR_H
 
+#include "hwasan.h"
+#include "hwasan_interface_internal.h"
+#include "hwasan_poisoning.h"
 #include "sanitizer_common/sanitizer_allocator.h"
 #include "sanitizer_common/sanitizer_allocator_checks.h"
 #include "sanitizer_common/sanitizer_allocator_interface.h"
 #include "sanitizer_common/sanitizer_allocator_report.h"
 #include "sanitizer_common/sanitizer_common.h"
 #include "sanitizer_common/sanitizer_ring_buffer.h"
-#include "hwasan_poisoning.h"
 
 #if !defined(__aarch64__) && !defined(__x86_64__)
 #error Unsupported platform
@@ -102,6 +104,11 @@
 
 void GetAllocatorStats(AllocatorStatCounters s);
 
+inline bool InTaggableRegion(uptr addr) {
+  // TODO: specialize for x86 once we use aliasing mode in the allocator.
+  return true;
+}
+
 } // namespace __hwasan
 
 #endif // HWASAN_ALLOCATOR_H
diff --git a/compiler-rt/lib/hwasan/hwasan_allocator.cpp b/compiler-rt/lib/hwasan/hwasan_allocator.cpp
--- a/compiler-rt/lib/hwasan/hwasan_allocator.cpp
+++ b/compiler-rt/lib/hwasan/hwasan_allocator.cpp
@@ -29,8 +29,8 @@
 static SpinMutex fallback_mutex;
 static atomic_uint8_t hwasan_allocator_tagging_enabled;
 
-static const tag_t kFallbackAllocTag = 0xBB;
-static const tag_t kFallbackFreeTag = 0xBC;
+static constexpr tag_t kFallbackAllocTag = 0xBB & kTagMask;
+static constexpr tag_t kFallbackFreeTag = 0xBC;
 
 enum RightAlignMode {
   kRightAlignNever,
@@ -149,7 +149,8 @@
   // false. When tag_in_malloc = false and tag_in_free = true malloc needs to
   // retag to 0.
   if ((flags()->tag_in_malloc || flags()->tag_in_free) &&
-      atomic_load_relaxed(&hwasan_allocator_tagging_enabled)) {
+      atomic_load_relaxed(&hwasan_allocator_tagging_enabled) &&
+      InTaggableRegion(reinterpret_cast<uptr>(user_ptr))) {
     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;
@@ -175,6 +176,8 @@
 static bool PointerAndMemoryTagsMatch(void *tagged_ptr) {
   CHECK(tagged_ptr);
   uptr tagged_uptr = reinterpret_cast<uptr>(tagged_ptr);
+  if (!InTaggableRegion(tagged_uptr))
+    return true;
   tag_t mem_tag = *reinterpret_cast<tag_t *>(
       MemToShadow(reinterpret_cast<uptr>(UntagPtr(tagged_ptr))));
   return PossiblyShortTagMatches(mem_tag, tagged_uptr, 1);
@@ -187,7 +190,9 @@
   if (!PointerAndMemoryTagsMatch(tagged_ptr))
     ReportInvalidFree(stack, reinterpret_cast<uptr>(tagged_ptr));
 
-  void *untagged_ptr = UntagPtr(tagged_ptr);
+  void *untagged_ptr = InTaggableRegion(reinterpret_cast<uptr>(tagged_ptr))
+                           ? UntagPtr(tagged_ptr)
+                           : tagged_ptr;
   void *aligned_ptr = reinterpret_cast<void *>(
       RoundDownTo(reinterpret_cast<uptr>(untagged_ptr), kShadowAlignment));
   Metadata *meta =
@@ -220,9 +225,13 @@
     internal_memset(aligned_ptr, flags()->free_fill_byte, fill_size);
   }
   if (flags()->tag_in_free && malloc_bisect(stack, 0) &&
-      atomic_load_relaxed(&hwasan_allocator_tagging_enabled))
+      atomic_load_relaxed(&hwasan_allocator_tagging_enabled) &&
+      InTaggableRegion(reinterpret_cast<uptr>(tagged_ptr))) {
+    // Always store full 8-bit tags on free to maximize UAF detection.
+    tag_t tag = t ? t->GenerateRandomTag(/*num_bits=*/8) : kFallbackFreeTag;
     TagMemoryAligned(reinterpret_cast<uptr>(aligned_ptr), TaggedSize(orig_size),
-                     t ? t->GenerateRandomTag() : kFallbackFreeTag);
+                     tag);
+  }
   if (t) {
     allocator.Deallocate(t->allocator_cache(), aligned_ptr);
     if (auto *ha = t->heap_allocations())
diff --git a/compiler-rt/lib/hwasan/hwasan_checks.h b/compiler-rt/lib/hwasan/hwasan_checks.h
--- a/compiler-rt/lib/hwasan/hwasan_checks.h
+++ b/compiler-rt/lib/hwasan/hwasan_checks.h
@@ -13,6 +13,7 @@
 #ifndef HWASAN_CHECKS_H
 #define HWASAN_CHECKS_H
 
+#include "hwasan_allocator.h"
 #include "hwasan_mapping.h"
 #include "sanitizer_common/sanitizer_common.h"
 
@@ -81,6 +82,8 @@
 
 template <ErrorAction EA, AccessType AT, unsigned LogSize>
 __attribute__((always_inline, nodebug)) static void CheckAddress(uptr p) {
+  if (!InTaggableRegion(p))
+    return;
   uptr ptr_raw = p & ~kAddressTagMask;
   tag_t mem_tag = *(tag_t *)MemToShadow(ptr_raw);
   if (UNLIKELY(!PossiblyShortTagMatches(mem_tag, p, 1 << LogSize))) {
@@ -94,7 +97,7 @@
 template <ErrorAction EA, AccessType AT>
 __attribute__((always_inline, nodebug)) static void CheckAddressSized(uptr p,
                                                                       uptr sz) {
-  if (sz == 0)
+  if (sz == 0 || !InTaggableRegion(p))
     return;
   tag_t ptr_tag = GetTagFromPointer(p);
   uptr ptr_raw = p & ~kAddressTagMask;
diff --git a/compiler-rt/lib/hwasan/hwasan_linux.cpp b/compiler-rt/lib/hwasan/hwasan_linux.cpp
--- a/compiler-rt/lib/hwasan/hwasan_linux.cpp
+++ b/compiler-rt/lib/hwasan/hwasan_linux.cpp
@@ -222,7 +222,9 @@
 }
 
 bool MemIsApp(uptr p) {
+#if !defined(__x86_64__)  // Aliasing-mode base tags are non-zero.
   CHECK(GetTagFromPointer(p) == 0);
+#endif
   return p >= kHighMemStart || (p >= kLowMemStart && p <= kLowMemEnd);
 }