LibJS: Never give back virtual memory once it belongs to a cell type

Instead of returning HeapBlock memory to the kernel (or a non-type
specific shared cache), we now keep a BlockAllocator per CellAllocator
and implement "deallocation" by basically informing the kernel that we
don't need the physical memory right now.

This is done with MADV_FREE or MADV_DONTNEED if available, but for other
platforms (including SerenityOS) we munmap and then re-mmap the memory
to achieve the same effect. It's definitely clunky, so I've added a
FIXME about implementing the madvise options on SerenityOS too.

The important outcome of this change is that GC types that use a
type-specific allocator become immune to use-after-free type confusion
attacks, since their virtual addresses will only ever be re-used for
the same exact type again and again.

Fixes #22274

Userland/Applications/Assistant/main.cpp

@@ -146,7 +146,7 @@ private:
 
 ErrorOr<int> serenity_main(Main::Arguments arguments)
 {
-    TRY(Core::System::pledge("stdio recvfd sendfd rpath cpath unix proc exec thread"));
+    TRY(Core::System::pledge("stdio recvfd sendfd rpath cpath unix proc exec thread map_fixed"));
 
     Core::LockFile lockfile("/tmp/lock/assistant.lock");
 

Userland/Applications/Spreadsheet/main.cpp

@@ -23,7 +23,7 @@
 
 ErrorOr<int> serenity_main(Main::Arguments arguments)
 {
-    TRY(Core::System::pledge("stdio recvfd sendfd rpath fattr unix cpath wpath thread"));
+    TRY(Core::System::pledge("stdio recvfd sendfd rpath fattr unix cpath wpath thread map_fixed"));
 
     auto app = TRY(GUI::Application::create(arguments));
 

Userland/Libraries/LibJS/Heap/BlockAllocator.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021-2023, Andreas Kling <kling@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -15,20 +15,24 @@
 #    include <sanitizer/asan_interface.h>
 #endif
 
+// FIXME: Implement MADV_FREE and/or MADV_DONTNEED on SerenityOS.
+#if defined(AK_OS_SERENITY) || (!defined(MADV_FREE) && !defined(MADV_DONTNEED))
+#    define USE_FALLBACK_BLOCK_DEALLOCATION
+#endif
+
 namespace JS {
 
+// NOTE: If this changes, we need to update the mmap() code to ensure correct alignment.
+static_assert(HeapBlock::block_size == 4096);
+
 BlockAllocator::~BlockAllocator()
 {
     for (auto* block : m_blocks) {
         ASAN_UNPOISON_MEMORY_REGION(block, HeapBlock::block_size);
-#ifdef AK_OS_SERENITY
         if (munmap(block, HeapBlock::block_size) < 0) {
             perror("munmap");
             VERIFY_NOT_REACHED();
         }
-#else
-        free(block);
-#endif
     }
 }
 
@@ -50,28 +54,38 @@ void* BlockAllocator::allocate_block([[maybe_unused]] char const* name)
 
 #ifdef AK_OS_SERENITY
     auto* block = (HeapBlock*)serenity_mmap(nullptr, HeapBlock::block_size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_RANDOMIZED | MAP_PRIVATE, 0, 0, HeapBlock::block_size, name);
-    VERIFY(block != MAP_FAILED);
 #else
-    auto* block = (HeapBlock*)aligned_alloc(HeapBlock::block_size, HeapBlock::block_size);
-    VERIFY(block);
+    auto* block = (HeapBlock*)mmap(nullptr, HeapBlock::block_size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
 #endif
+    VERIFY(block != MAP_FAILED);
     return block;
 }
 
 void BlockAllocator::deallocate_block(void* block)
 {
     VERIFY(block);
-    if (m_blocks.size() >= max_cached_blocks) {
-#ifdef AK_OS_SERENITY
+
+#if defined(USE_FALLBACK_BLOCK_DEALLOCATION)
+    // If we can't use any of the nicer techniques, unmap and remap the block to return the physical pages while keeping the VM.
     if (munmap(block, HeapBlock::block_size) < 0) {
         perror("munmap");
         VERIFY_NOT_REACHED();
     }
-#else
-        free(block);
-#endif
-        return;
+    if (mmap(block, HeapBlock::block_size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, 0, 0) != block) {
+        perror("mmap");
+        VERIFY_NOT_REACHED();
     }
+#elif defined(MADV_FREE)
+    if (madvise(block, HeapBlock::block_size, MADV_FREE) < 0) {
+        perror("madvise(MADV_FREE)");
+        VERIFY_NOT_REACHED();
+    }
+#elif defined(MADV_DONTNEED)
+    if (madvise(block, HeapBlock::block_size, MADV_DONTNEED) < 0) {
+        perror("madvise(MADV_DONTNEED)");
+        VERIFY_NOT_REACHED();
+    }
+#endif
 
     ASAN_POISON_MEMORY_REGION(block, HeapBlock::block_size);
     m_blocks.append(block);

Userland/Libraries/LibJS/Heap/BlockAllocator.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021-2023, Andreas Kling <kling@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -20,9 +20,7 @@ public:
     void deallocate_block(void*);
 
 private:
-    static constexpr size_t max_cached_blocks = 512;
-
-    Vector<void*, max_cached_blocks> m_blocks;
+    Vector<void*> m_blocks;
 };
 
 }

Userland/Libraries/LibJS/Heap/CellAllocator.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2020-2023, Andreas Kling <kling@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -37,11 +37,10 @@ Cell* CellAllocator::allocate_cell(Heap& heap)
 
 void CellAllocator::block_did_become_empty(Badge<Heap>, HeapBlock& block)
 {
-    auto& heap = block.heap();
     block.m_list_node.remove();
     // NOTE: HeapBlocks are managed by the BlockAllocator, so we don't want to `delete` the block here.
     block.~HeapBlock();
-    heap.block_allocator().deallocate_block(&block);
+    m_block_allocator.deallocate_block(&block);
 }
 
 void CellAllocator::block_did_become_usable(Badge<Heap>, HeapBlock& block)

Userland/Libraries/LibJS/Heap/CellAllocator.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2020-2023, Andreas Kling <kling@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -10,6 +10,7 @@
 #include <AK/NeverDestroyed.h>
 #include <AK/NonnullOwnPtr.h>
 #include <LibJS/Forward.h>
+#include <LibJS/Heap/BlockAllocator.h>
 #include <LibJS/Heap/HeapBlock.h>
 
 #define JS_DECLARE_ALLOCATOR(ClassName) \
@@ -49,9 +50,13 @@ public:
     IntrusiveListNode<CellAllocator> m_list_node;
     using List = IntrusiveList<&CellAllocator::m_list_node>;
 
+    BlockAllocator& block_allocator() { return m_block_allocator; }
+
 private:
     size_t const m_cell_size;
 
+    BlockAllocator m_block_allocator;
+
     using BlockList = IntrusiveList<&HeapBlock::m_list_node>;
     BlockList m_full_blocks;
     BlockList m_usable_blocks;

Userland/Libraries/LibJS/Heap/Heap.h

@@ -15,7 +15,6 @@
 #include <AK/Vector.h>
 #include <LibCore/Forward.h>
 #include <LibJS/Forward.h>
-#include <LibJS/Heap/BlockAllocator.h>
 #include <LibJS/Heap/Cell.h>
 #include <LibJS/Heap/CellAllocator.h>
 #include <LibJS/Heap/Handle.h>
@@ -83,8 +82,6 @@ public:
 
     void register_cell_allocator(Badge<CellAllocator>, CellAllocator&);
 
-    BlockAllocator& block_allocator() { return m_block_allocator; }
-
     void uproot_cell(Cell* cell);
 
 private:
@@ -154,8 +151,6 @@ private:
 
     Vector<GCPtr<Cell>> m_uprooted_cells;
 
-    BlockAllocator m_block_allocator;
-
     size_t m_gc_deferrals { 0 };
     bool m_should_gc_when_deferral_ends { false };
 

Userland/Libraries/LibJS/Heap/HeapBlock.cpp

@@ -26,7 +26,7 @@ NonnullOwnPtr<HeapBlock> HeapBlock::create_with_cell_size(Heap& heap, CellAlloca
 #else
     char const* name = nullptr;
 #endif
-    auto* block = static_cast<HeapBlock*>(heap.block_allocator().allocate_block(name));
+    auto* block = static_cast<HeapBlock*>(cell_allocator.block_allocator().allocate_block(name));
     new (block) HeapBlock(heap, cell_allocator, cell_size);
     return NonnullOwnPtr<HeapBlock>(NonnullOwnPtr<HeapBlock>::Adopt, *block);
 }

Userland/Services/WebContent/main.cpp

@@ -27,7 +27,7 @@
 ErrorOr<int> serenity_main(Main::Arguments)
 {
     Core::EventLoop event_loop;
-    TRY(Core::System::pledge("stdio recvfd sendfd accept unix rpath thread proc"));
+    TRY(Core::System::pledge("stdio recvfd sendfd accept unix rpath thread proc map_fixed"));
 
     // This must be first; we can't check if /tmp/webdriver exists once we've unveiled other paths.
     auto webdriver_socket_path = ByteString::formatted("{}/webdriver", TRY(Core::StandardPaths::runtime_directory()));

Userland/Utilities/js.cpp

@@ -531,7 +531,7 @@ private:
 
 ErrorOr<int> serenity_main(Main::Arguments arguments)
 {
-    TRY(Core::System::pledge("stdio rpath wpath cpath tty sigaction"));
+    TRY(Core::System::pledge("stdio rpath wpath cpath tty sigaction map_fixed"));
 
     bool gc_on_every_allocation = false;
     bool disable_syntax_highlight = false;