Bug 1402282 - Change jemalloc to use secure random private arena ids r=glandium

Previously the id for a new arena was just a counter that increased by one
every time. For hardening purposes, we want to make private arenas use a secure
random ID, so an attacker will have a more difficult time finding the memory
they are looking for.

Differential Revision: https://phabricator.services.mozilla.com/D10158

--HG--
extra : moz-landing-system : lando

memory/build/mozjemalloc.cpp

@@ -132,6 +132,7 @@
 #include "mozilla/DoublyLinkedList.h"
 #include "mozilla/Likely.h"
 #include "mozilla/MathAlgorithms.h"
+#include "mozilla/RandomNum.h"
 #include "mozilla/Sprintf.h"
 // Note: MozTaggedAnonymousMmap() could call an LD_PRELOADed mmap
 // instead of the one defined here; use only MozTagAnonymousMemory().
@@ -1158,8 +1159,10 @@ public:
   Mutex mLock;
 
 private:
+  inline arena_t* GetByIdInternal(arena_id_t aArenaId, bool aIsPrivate);
+
   arena_t* mDefaultArena;
-  arena_id_t mLastArenaId;
+  arena_id_t mLastPublicArenaId;
   Tree mArenas;
   Tree mPrivateArenas;
 };
@@ -3734,10 +3737,34 @@ ArenaCollection::CreateArena(bool aIsPrivate, arena_params_t* aParams)
 
   MutexAutoLock lock(mLock);
 
-  // TODO: Use random Ids.
-  ret->mId = mLastArenaId++;
-  (aIsPrivate ? mPrivateArenas : mArenas).Insert(ret);
-  return ret;
+  // For public arenas, it's fine to just use incrementing arena id
+  if (!aIsPrivate) {
+    ret->mId = mLastPublicArenaId++;
+    mArenas.Insert(ret);
+    return ret;
+  }
+
+  // For private arenas, generate a cryptographically-secure random id for the 
+  // new arena. If an attacker manages to get control of the process, this 
+  // should make it more difficult for them to "guess" the ID of a memory 
+  // arena, stopping them from getting data they may want
+
+  while(true) {
+    mozilla::Maybe<uint64_t> maybeRandomId = mozilla::RandomUint64();
+    MOZ_RELEASE_ASSERT(maybeRandomId.isSome());
+
+    // Keep looping until we ensure that the random number we just generated
+    // isn't already in use by another active arena
+    arena_t* existingArena =
+      GetByIdInternal(maybeRandomId.value(), true /*aIsPrivate*/);
+
+    if (!existingArena) {
+      ret->mId = static_cast<arena_id_t>(maybeRandomId.value());
+      mPrivateArenas.Insert(ret);
+      return ret;
+    }
+  }
+
 }
 
 // End arena.
@@ -4561,18 +4588,25 @@ MozJemalloc::jemalloc_free_dirty_pages(void)
   }
 }
 
+inline arena_t*
+ArenaCollection::GetByIdInternal(arena_id_t aArenaId, bool aIsPrivate)
+{
+  // Use AlignedStorage2 to avoid running the arena_t constructor, while
+  // we only need it as a placeholder for mId.
+  mozilla::AlignedStorage2<arena_t> key;
+  key.addr()->mId = aArenaId;
+  return (aIsPrivate ? mPrivateArenas : mArenas).Search(key.addr());
+}
+
 inline arena_t*
 ArenaCollection::GetById(arena_id_t aArenaId, bool aIsPrivate)
 {
   if (!malloc_initialized) {
     return nullptr;
   }
-  // Use AlignedStorage2 to avoid running the arena_t constructor, while
-  // we only need it as a placeholder for mId.
-  mozilla::AlignedStorage2<arena_t> key;
-  key.addr()->mId = aArenaId;
+  
   MutexAutoLock lock(mLock);
-  arena_t* result = (aIsPrivate ? mPrivateArenas : mArenas).Search(key.addr());
+  arena_t* result = GetByIdInternal(aArenaId, aIsPrivate);
   MOZ_RELEASE_ASSERT(result);
   return result;
 }

memory/replace/logalloc/replay/Replay.cpp

@@ -300,12 +300,6 @@ MOZ_BEGIN_EXTERN_C
 #include "malloc_decls.h"
 
 #ifdef ANDROID
-/* mozjemalloc uses MozTagAnonymousMemory, which doesn't have an inline
- * implementation on Android */
-void
-MozTagAnonymousMemory(const void* aPtr, size_t aLength, const char* aTag)
-{
-}
 
 /* mozjemalloc and jemalloc use pthread_atfork, which Android doesn't have.
  * While gecko has one in libmozglue, the replay program can't use that.

memory/replace/logalloc/replay/moz.build

@@ -8,6 +8,9 @@ Program('logalloc-replay')
 
 SOURCES += [
     '/mfbt/Assertions.cpp',
+    '/mfbt/Poison.cpp',
+    '/mfbt/RandomNum.cpp',
+    '/mfbt/TaggedAnonymousMemory.cpp',
     '/mfbt/Unused.cpp',
     'Replay.cpp',
 ]
@@ -16,7 +19,6 @@ if CONFIG['MOZ_REPLACE_MALLOC_STATIC'] and CONFIG['MOZ_DMD']:
     UNIFIED_SOURCES += [
         '/mfbt/HashFunctions.cpp',
         '/mfbt/JSONWriter.cpp',
-        '/mfbt/Poison.cpp',
         '/mozglue/misc/StackWalk.cpp',
     ]