Scudo Hardened Allocator

Introduction

The Scudo Hardened Allocator is a user-mode allocator, originally based on LLVM Sanitizers’ CombinedAllocator. It aims at providing additional mitigation against heap based vulnerabilities, while maintaining good performance. Scudo is currently the default allocator in Fuchsia, and in Android since Android 11.

The name “Scudo” comes from the Italian word for shield (and Escudo in Spanish).

Design

Allocator

Scudo was designed with security in mind, but aims at striking a good balance between security and performance. It was designed to be highly tunable and configurable, and while we provide some default configurations, we encourage consumers to come up with the parameters that will work best for their use cases.

The allocator combines several components that serve distinct purposes:

  • the Primary allocator: fast and efficient, it services smaller allocation sizes by carving reserved memory regions into blocks of identical size. There are currently two Primary allocators implemented, specific to 32 and 64 bit architectures. It is configurable via compile time options.
  • the Secondary allocator: slower, it services larger allocation sizes via the memory mapping primitives of the underlying operating system. Secondary backed allocations are surrounded by Guard Pages. It is also configurable via compile time options.
  • the thread specific data Registry: defines how local caches operate for each thread. There are currently two models implemented: the exclusive model where each thread holds its own caches (using the ELF TLS); or the shared model where threads share a fixed size pool of caches.
  • the Quarantine: offers a way to delay the deallocation operations, preventing blocks to be immediately available for reuse. Blocks held will be recycled once certain size criteria are reached. This is essentially a delayed freelist which can help mitigate some use-after-free situations. This feature is fairly costly in terms of performance and memory footprint, is mostly controlled by runtime options and is disabled by default.

Allocations Header

Every chunk of heap memory returned to an application by the allocator will be preceded by a header. This has two purposes:

  • being to store various information about the chunk, that can be leveraged to ensure consistency of the heap operations;
  • being able to detect potential corruption. For this purpose, the header is checksummed and corruption of the header will be detected when said header is accessed (note that if the corrupted header is not accessed, the corruption will remain undetected).

The following information is stored in the header:

  • the class ID for that chunk, which identifies the region where the chunk resides for Primary backed allocations, or 0 for Secondary backed allocations;
  • the state of the chunk (available, allocated or quarantined);
  • the allocation type (malloc, new, new[] or memalign), to detect potential mismatches in the allocation APIs used;
  • the size (Primary) or unused bytes amount (Secondary) for that chunk, which is necessary for reallocation or sized-deallocation operations;
  • the offset of the chunk, which is the distance in bytes from the beginning of the returned chunk to the beginning of the backend allocation (the “block”);
  • the 16-bit checksum;

This header fits within 8 bytes on all platforms supported, and contributes to a small overhead for each allocation.

The checksum is computed using a CRC32 (made faster with hardware support) of the global secret, the chunk pointer itself, and the 8 bytes of header with the checksum field zeroed out. It is not intended to be cryptographically strong.

The header is atomically loaded and stored to prevent races. This is important as two consecutive chunks could belong to different threads. We work on local copies and use compare-exchange primitives to update the headers in the heap memory, and avoid any type of double-fetching.

Randomness

Randomness is a critical factor to the additional security provided by the allocator. The allocator trusts the memory mapping primitives of the OS to provide pages at (mostly) non-predictable locations in memory, as well as the binaries to be compiled with ASLR. In the event one of those assumptions is incorrect, the security will be greatly reduced. Scudo further randomizes how blocks are allocated in the Primary, can randomize how caches are assigned to threads.

Memory reclaiming

Primary and Secondary allocators have different behaviors with regard to reclaiming. While Secondary mapped allocations can be unmapped on deallocation, it isn’t the case for the Primary, which could lead to a steady growth of the RSS of a process. To counteracty this, if the underlying OS allows it, pages that are covered by contiguous free memory blocks in the Primary can be released: this generally means they won’t count towards the RSS of a process and be zero filled on subsequent accesses). This is done in the deallocation path, and several options exist to tune this behavior.

Usage

Platform

If using Fuchsia or an Android version greater than 11, your memory allocations are already service by Scudo (note that Android Svelte configurations still use jemalloc).

Library

The allocator static library can be built from the LLVM tree thanks to the scudo_standalone CMake rule. The associated tests can be exercised thanks to the check-scudo_standalone CMake rule.

Linking the static library to your project can require the use of the whole-archive linker flag (or equivalent), depending on your linker. Additional flags might also be necessary.

Your linked binary should now make use of the Scudo allocation and deallocation functions.

You may also build Scudo like this:

cd $LLVM/compiler-rt/lib
clang++ -fPIC -std=c++17 -msse4.2 -O2 -pthread -shared \
  -I scudo/standalone/include \
  scudo/standalone/*.cpp \
  -o $HOME/libscudo.so

and then use it with existing binaries as follows:

LD_PRELOAD=$HOME/libscudo.so ./a.out

Clang

With a recent version of Clang (post rL317337), the “old” version of the allocator can be linked with a binary at compilation using the -fsanitize=scudo command-line argument, if the target platform is supported. Currently, the only other sanitizer Scudo is compatible with is UBSan (eg: -fsanitize=scudo,undefined). Compiling with Scudo will also enforce PIE for the output binary.

We will transition this to the standalone Scudo version in the future.

Options

Several aspects of the allocator can be configured on a per process basis through the following ways:

  • at compile time, by defining SCUDO_DEFAULT_OPTIONS to the options string you want set by default;
  • by defining a __scudo_default_options function in one’s program that returns the options string to be parsed. Said function must have the following prototype: extern "C" const char* __scudo_default_options(void), with a default visibility. This will override the compile time define;
  • through the environment variable SCUDO_OPTIONS, containing the options string to be parsed. Options defined this way will override any definition made through __scudo_default_options.
  • via the standard mallopt API, using parameters that are Scudo specific.

When dealing with the options string, it follows a syntax similar to ASan, where distinct options can be assigned in the same string, separated by colons.

For example, using the environment variable:

SCUDO_OPTIONS="delete_size_mismatch=false:release_to_os_interval_ms=-1" ./a.out

Or using the function:

extern "C" const char *__scudo_default_options() {
  return "delete_size_mismatch=false:release_to_os_interval_ms=-1";
}

The following “string” options are available:

Option 64-bit default 32-bit default Description
quarantine_size_kb 0 0 The size (in Kb) of quarantine used to delay the actual deallocation of chunks. Lower value may reduce memory usage but decrease the effectiveness of the mitigation; a negative value will fallback to the defaults. Setting both this and thread_local_quarantine_size_kb to zero will disable the quarantine entirely.
quarantine_max_chunk_size 0 0 Size (in bytes) up to which chunks can be quarantined.
thread_local_quarantine_size_kb 0 0 The size (in Kb) of per-thread cache use to offload the global quarantine. Lower value may reduce memory usage but might increase contention on the global quarantine. Setting both this and quarantine_size_kb to zero will disable the quarantine entirely.
dealloc_type_mismatch false false Whether or not we report errors on malloc/delete, new/free, new/delete[], etc.
delete_size_mismatch true true Whether or not we report errors on mismatch between sizes of new and delete.
zero_contents false false Whether or not we zero chunk contents on allocation.
pattern_fill_contents false false Whether or not we fill chunk contents with a byte pattern on allocation.
may_return_null true true Whether or not a non-fatal failure can return a NULL pointer (as opposed to terminating).
release_to_os_interval_ms 5000 5000 The minimum interval (in ms) at which a release can be attempted (a negative value disables reclaiming).

Additional flags can be specified, for example if Scudo if compiled with GWP-ASan support.

The following “mallopt” options are available (options are defined in include/scudo/interface.h):

Option Description
M_DECAY_TIME Sets the release interval option to the specified value (Android only allows 0 or 1 to respectively set the interval to the minimum and maximum value as specified at compile time).
M_PURGE Forces immediate memory reclaiming (value is unused).
M_MEMTAG_TUNING Tunes the allocator’s choice of memory tags to make it more likely that a certain class of memory errors will be detected. The value argument should be one of the enumerators of scudo_memtag_tuning.
M_THREAD_DISABLE_MEM_INIT Tunes the per-thread memory initialization, 0 being the normal behavior, 1 disabling the automatic heap initialization.
M_CACHE_COUNT_MAX Set the maximum number of entries than can be cached in the Secondary cache.
M_CACHE_SIZE_MAX Sets the maximum size of entries that can be cached in the Secondary cache.
M_TSDS_COUNT_MAX Increases the maximum number of TSDs that can be used up to the limit specified at compile time.

Error Types

The allocator will output an error message, and potentially terminate the process, when an unexpected behavior is detected. The output usually starts with "Scudo ERROR:" followed by a short summary of the problem that occurred as well as the pointer(s) involved. Once again, Scudo is meant to be a mitigation, and might not be the most useful of tools to help you root-cause the issue, please consider ASan for this purpose.

Here is a list of the current error messages and their potential cause:

  • "corrupted chunk header": the checksum verification of the chunk header has failed. This is likely due to one of two things: the header was overwritten (partially or totally), or the pointer passed to the function is not a chunk at all;
  • "race on chunk header": two different threads are attempting to manipulate the same header at the same time. This is usually symptomatic of a race-condition or general lack of locking when performing operations on that chunk;
  • "invalid chunk state": the chunk is not in the expected state for a given operation, eg: it is not allocated when trying to free it, or it’s not quarantined when trying to recycle it, etc. A double-free is the typical reason this error would occur;
  • "misaligned pointer": we strongly enforce basic alignment requirements, 8 bytes on 32-bit platforms, 16 bytes on 64-bit platforms. If a pointer passed to our functions does not fit those, something is definitely wrong.
  • "allocation type mismatch": when the optional deallocation type mismatch check is enabled, a deallocation function called on a chunk has to match the type of function that was called to allocate it. Security implications of such a mismatch are not necessarily obvious but situational at best;
  • "invalid sized delete": when the C++14 sized delete operator is used, and the optional check enabled, this indicates that the size passed when deallocating a chunk is not congruent with the one requested when allocating it. This is likely to be a compiler issue, as was the case with Intel C++ Compiler, or some type confusion on the object being deallocated;
  • "RSS limit exhausted": the maximum RSS optionally specified has been exceeded;

Several other error messages relate to parameter checking on the libc allocation APIs and are fairly straightforward to understand.