[hwasan] Fix false positive when tag_in_malloc=0,tag_in_free=1.

[eugenis]

Summary

With tag_in_free=1, malloc() can not assume that the memory is untagged,
and needs to retag is to 0.

[eugenis]

+ a comment in test

[pcc]

LGTM

lib/hwasan/hwasan_allocator.cc

if (zeroise) {
     uptr fill_size = Min(size, (uptr)flags()->max_malloc_fill_size);
     internal_memset(allocated, flags()->malloc_fill_byte, fill_size);
   }
   if (!right_align_mode)
     internal_memcpy(reinterpret_cast<u8 *>(allocated) + orig_size, tail_magic,
                     size - orig_size);
 
   void *user_ptr = allocated;
-  if (flags()->tag_in_malloc &&
-      atomic_load_relaxed(&hwasan_allocator_tagging_enabled))
-    user_ptr = (void *)TagMemoryAligned(
-        (uptr)user_ptr, size, t ? t->GenerateRandomTag() : kFallbackAllocTag);
+  // Tagging can only be skipped when both tag_in_malloc and tag_in_free are
+  // 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)) {
+    tag_t tag = flags()->tag_in_malloc
+                    ? (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;

test/hwasan/TestCases/tag_in_free.c

+// RUN: %clang_hwasan -O0 %s -DMALLOC -DFREE -o %t.mf
+// RUN: %env_hwasan_opts=tag_in_malloc=0,tag_in_free=1 not %run %t.mf 2>&1 | FileCheck %s --check-prefixes=FREE
+// RUN: %env_hwasan_opts=tag_in_malloc=1,tag_in_free=1 not %run %t.mf 2>&1 | FileCheck %s --check-prefixes=MALLOC
+// RUN: %env_hwasan_opts=tag_in_malloc=1,tag_in_free=0 not %run %t.mf 2>&1 | FileCheck %s --check-prefixes=MALLOC
+// RUN: %env_hwasan_opts=tag_in_malloc=0,tag_in_free=0     %run %t.mf 2>&1
+
+// RUN: %clang_hwasan -O0 %s -DFREE -o %t.f
+// RUN: %env_hwasan_opts=tag_in_malloc=0,tag_in_free=1 not %run %t.f 2>&1 | FileCheck %s --check-prefixes=FREE
+// RUN: %env_hwasan_opts=tag_in_malloc=1,tag_in_free=1 not %run %t.f 2>&1 | FileCheck %s --check-prefixes=FREE
+// RUN: %env_hwasan_opts=tag_in_malloc=1,tag_in_free=0     %run %t.f 2>&1
+// RUN: %env_hwasan_opts=tag_in_malloc=0,tag_in_free=0     %run %t.f 2>&1
+
+// RUN: %clang_hwasan -O0 %s -DMALLOC -o %t.m
+// RUN: %env_hwasan_opts=tag_in_malloc=0,tag_in_free=1     %run %t.m 2>&1
+// RUN: %env_hwasan_opts=tag_in_malloc=1,tag_in_free=1 not %run %t.m 2>&1 | FileCheck %s --check-prefixes=MALLOC
+// RUN: %env_hwasan_opts=tag_in_malloc=1,tag_in_free=0 not %run %t.m 2>&1 | FileCheck %s --check-prefixes=MALLOC
+// RUN: %env_hwasan_opts=tag_in_malloc=0,tag_in_free=0     %run %t.m 2>&1
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <sanitizer/hwasan_interface.h>
+
+int main() {
+  __hwasan_enable_allocator_tagging();
+  // Loop for a while to make sure that the memory for the test below is reused after an earlier free(),
+  // and is potentially tagged (when tag_in_free == 1).
+  for (int i = 0; i < 100; ++i) {
+    char * volatile p = (char*)malloc(10);
+    free(p);
+  }
+
+  char * volatile p = (char*)malloc(10);
+#ifdef MALLOC
+  // MALLOC: READ of size 1 at
+  // MALLOC: is located 6 bytes to the right of 10-byte region
+  // MALLOC: allocated here:
+  char volatile x = p[16];
+#endif
+  free(p);
+#ifdef FREE
+  // FREE: READ of size 1 at
+  // FREE: is located 0 bytes inside of 10-byte region
+  // FREE: freed by thread T0 here:
+  // FREE: previously allocated here:
+  char volatile y = p[0];
+#endif
+
+  __hwasan_disable_allocator_tagging();
+
+  return 0;
+}