diff --git a/util/posix/scoped_mmap.cc b/util/posix/scoped_mmap.cc
index f8449b1f..4860cecb 100644
--- a/util/posix/scoped_mmap.cc
+++ b/util/posix/scoped_mmap.cc
@@ -19,6 +19,7 @@
 #include <algorithm>
 
 #include "base/logging.h"
+#include "base/numerics/safe_math.h"
 
 namespace {
 
@@ -56,6 +57,7 @@ bool ScopedMmap::ResetAddrLen(void* addr, size_t len) {
     DCHECK_NE(len, 0u);
     DCHECK_EQ(new_addr % getpagesize(), 0u);
     DCHECK_EQ(len % getpagesize(), 0u);
+    DCHECK((base::CheckedNumeric<uintptr_t>(new_addr) + (len - 1)).IsValid());
   }
 
   bool result = true;
diff --git a/util/posix/scoped_mmap_test.cc b/util/posix/scoped_mmap_test.cc
index ea96c65f..0c4787b2 100644
--- a/util/posix/scoped_mmap_test.cc
+++ b/util/posix/scoped_mmap_test.cc
@@ -302,11 +302,15 @@ TEST(ScopedMmapDeathTest, Mprotect) {
   EXPECT_EQ(kPageSize, mapping.len());
 
   char* addr = mapping.addr_as<char*>();
-  *addr = 0;
+  *addr = 1;
 
   ASSERT_TRUE(mapping.Mprotect(PROT_READ));
 
   EXPECT_DEATH(*addr = 0, "");
+
+  ASSERT_TRUE(mapping.Mprotect(PROT_READ | PROT_WRITE));
+  EXPECT_EQ(1, *addr);
+  *addr = 2;
 }
 
 }  // namespace