Index: lib/ubsan/ubsan_type_hash_itanium.cc
===================================================================
--- lib/ubsan/ubsan_type_hash_itanium.cc
+++ lib/ubsan/ubsan_type_hash_itanium.cc
@@ -17,6 +17,7 @@
 #include "ubsan_type_hash.h"
 
 #include "sanitizer_common/sanitizer_common.h"
+#include "sanitizer_common/sanitizer_procmaps.h"
 
 // The following are intended to be binary compatible with the definitions
 // given in the Itanium ABI. We make no attempt to be ODR-compatible with
@@ -191,7 +192,45 @@
   /// The type_info object describing the most-derived class type.
   std::type_info *TypeInfo;
 };
+
+#if SANITIZER_LINUX
+bool isValidVptr(void *Vtable) {
+  // Validate the memory permissions of the vtable pointer and the first
+  // function pointer in the vtable. They should be r-- or r-x and r-x
+  // respectively. Only enabled for Linux; this hasn't been tested on FreeBSD,
+  // and vtables are writable on Mac (PR24782) so this won't work there.
+  uptr FirstFunctionPtr = *reinterpret_cast<uptr *>(Vtable);
+  bool ValidVtable = false, ValidFirstFunctionPtr = false;
+  MemoryMappingLayout Layout(/*cache_enabled=*/true);
+  uptr Start, End, Prot;
+  while (Layout.Next(&Start, &End, 0, 0, 0, &Prot)) {
+    if (Start <= ((uptr)Vtable) && ((uptr)Vtable) <= End &&
+        (Prot == MemoryMappingLayout::kProtectionRead ||
+         Prot == (MemoryMappingLayout::kProtectionRead |
+                  MemoryMappingLayout::kProtectionExecute)))
+      ValidVtable = true;
+    if (Start <= FirstFunctionPtr && FirstFunctionPtr <= End &&
+        Prot == (MemoryMappingLayout::kProtectionRead |
+                 MemoryMappingLayout::kProtectionExecute))
+      ValidFirstFunctionPtr = true;
+    if (ValidVtable && ValidFirstFunctionPtr)
+      return true;
+  }
+  return false;
+}
+#else  // !SANITIZER_LINUX
+bool isValidVptr(void *Vtable) {
+  return true;
+}
+#endif
+
 VtablePrefix *getVtablePrefix(void *Vtable) {
+  if (!IsAccessibleMemoryRange((uptr)Vtable, sizeof(void *)))
+    return 0;
+
+  if (!isValidVptr(Vtable))
+    return 0;
+
   VtablePrefix *Vptr = reinterpret_cast<VtablePrefix*>(Vtable);
   if (!Vptr)
     return 0;
Index: test/ubsan/TestCases/TypeCheck/vptr-bad-perms.cpp
===================================================================
--- /dev/null
+++ test/ubsan/TestCases/TypeCheck/vptr-bad-perms.cpp
@@ -0,0 +1,33 @@
+// RUN: %clangxx -frtti -fsanitize=vptr -fno-sanitize-recover=vptr -g %s -O3 -o %t
+// RUN: not %run %t 2>&1 | FileCheck %s
+
+// Tests that we consider vtable pointers in writable memory to be invalid.
+
+// REQUIRES: vptr-validation
+
+#include <string.h>
+
+struct A {
+  virtual void f();
+};
+
+void A::f() {}
+
+struct B {
+  virtual void f();
+};
+
+void B::f() {}
+
+int main() {
+  // Create a fake vtable for A in writable memory and copy A's vtable into it.
+  void *fake_vtable[3];
+  A a;
+  void ***vtp = (void ***)&a;
+  memcpy(fake_vtable, *vtp - 2, sizeof(void *) * 3);
+  *vtp = fake_vtable + 2;
+
+  // A's vtable is invalid because it lives in writable memory.
+  // CHECK: invalid vptr
+  reinterpret_cast<B*>(&a)->f();
+}
Index: test/ubsan/lit.common.cfg
===================================================================
--- test/ubsan/lit.common.cfg
+++ test/ubsan/lit.common.cfg
@@ -77,3 +77,6 @@
 # because the test hangs or fails on one configuration and not the other.
 if config.target_arch.startswith('arm') == False:
   config.available_features.add('stable-runtime')
+
+if config.host_os == 'Linux':
+  config.available_features.add('vptr-validation')