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168 lines
7.0 KiB
ReStructuredText
168 lines
7.0 KiB
ReStructuredText
========================
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Scudo Hardened Allocator
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========================
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.. contents::
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:local:
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:depth: 1
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Introduction
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============
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The Scudo Hardened Allocator is a user-mode allocator based on LLVM Sanitizer's
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CombinedAllocator, which aims at providing additional mitigations against heap
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based vulnerabilities, while maintaining good performance.
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The name "Scudo" has been retained from the initial implementation (Escudo
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meaning Shield in Spanish and Portuguese).
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Design
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======
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Chunk Header
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------------
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Every chunk of heap memory will be preceded by a chunk header. This has two
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purposes, the first one being to store various information about the chunk,
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the second one being to detect potential heap overflows. In order to achieve
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this, the header will be checksumed, involving the pointer to the chunk itself
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and a global secret. Any corruption of the header will be detected when said
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header is accessed, and the process terminated.
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The following information is stored in the header:
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- the 16-bit checksum;
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- the user requested size for that chunk, which is necessary for reallocation
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purposes;
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- the state of the chunk (available, allocated or quarantined);
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- the allocation type (malloc, new, new[] or memalign), to detect potential
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mismatches in the allocation APIs used;
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- whether or not the chunk is offseted (ie: if the chunk beginning is different
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than the backend allocation beginning, which is most often the case with some
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aligned allocations);
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- the associated offset;
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- a 16-bit salt.
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On x64, which is currently the only architecture supported, the header fits
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within 16-bytes, which works nicely with the minimum alignment requirements.
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The checksum is computed as a CRC32 (requiring the SSE 4.2 instruction set)
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of the global secret, the chunk pointer itself, and the 16 bytes of header with
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the checksum field zeroed out.
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The header is atomically loaded and stored to prevent races (this requires
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platform support such as the cmpxchg16b instruction). This is important as two
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consecutive chunks could belong to different threads. We also want to avoid
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any type of double fetches of information located in the header, and use local
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copies of the header for this purpose.
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Delayed Freelist
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-----------------
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A delayed freelist allows us to not return a chunk directly to the backend, but
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to keep it aside for a while. Once a criterion is met, the delayed freelist is
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emptied, and the quarantined chunks are returned to the backend. This helps
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mitigate use-after-free vulnerabilities by reducing the determinism of the
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allocation and deallocation patterns.
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This feature is using the Sanitizer's Quarantine as its base, and the amount of
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memory that it can hold is configurable by the user (see the Options section
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below).
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Randomness
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----------
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It is important for the allocator to not make use of fixed addresses. We use
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the dynamic base option for the SizeClassAllocator, allowing us to benefit
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from the randomness of mmap.
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Usage
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=====
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Library
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-------
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The allocator static library can be built from the LLVM build tree thanks to
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the ``scudo`` CMake rule. The associated tests can be exercised thanks to the
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``check-scudo`` CMake rule.
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Linking the static library to your project can require the use of the
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``whole-archive`` linker flag (or equivalent), depending on your linker.
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Additional flags might also be necessary.
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Your linked binary should now make use of the Scudo allocation and deallocation
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functions.
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You may also build Scudo like this:
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.. code::
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cd $LLVM/projects/compiler-rt/lib
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clang++ -fPIC -std=c++11 -msse4.2 -mcx16 -O2 -I. scudo/*.cpp \
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$(\ls sanitizer_common/*.{cc,S} | grep -v "sanitizer_termination\|sanitizer_common_nolibc") \
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-shared -o scudo-allocator.so -lpthread
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and then use it with existing binaries as follows:
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.. code::
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LD_PRELOAD=`pwd`/scudo-allocator.so ./a.out
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Options
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-------
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Several aspects of the allocator can be configured through the following ways:
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- by defining a ``__scudo_default_options`` function in one's program that
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returns the options string to be parsed. Said function must have the following
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prototype: ``extern "C" const char* __scudo_default_options()``.
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- through the environment variable SCUDO_OPTIONS, containing the options string
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to be parsed. Options defined this way will override any definition made
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through ``__scudo_default_options``;
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The options string follows a syntax similar to ASan, where distinct options
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can be assigned in the same string, separated by colons.
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For example, using the environment variable:
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.. code::
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SCUDO_OPTIONS="DeleteSizeMismatch=1:QuarantineSizeMb=16" ./a.out
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Or using the function:
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.. code::
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extern "C" const char *__scudo_default_options() {
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return "DeleteSizeMismatch=1:QuarantineSizeMb=16";
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}
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The following options are available:
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+-----------------------------+---------+------------------------------------------------+
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| Option | Default | Description |
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+-----------------------------+---------+------------------------------------------------+
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| QuarantineSizeMb | 64 | The size (in Mb) of quarantine used to delay |
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| | | the actual deallocation of chunks. Lower value |
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| | | may reduce memory usage but decrease the |
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| | | effectiveness of the mitigation; a negative |
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| | | value will fallback to a default of 64Mb. |
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+-----------------------------+---------+------------------------------------------------+
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| ThreadLocalQuarantineSizeKb | 1024 | The size (in Kb) of per-thread cache use to |
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| | | offload the global quarantine. Lower value may |
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| | | reduce memory usage but might increase |
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| | | contention on the global quarantine. |
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+-----------------------------+---------+------------------------------------------------+
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| DeallocationTypeMismatch | true | Whether or not we report errors on |
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| | | malloc/delete, new/free, new/delete[], etc. |
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+-----------------------------+---------+------------------------------------------------+
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| DeleteSizeMismatch | true | Whether or not we report errors on mismatch |
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| | | between sizes of new and delete. |
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+-----------------------------+---------+------------------------------------------------+
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| ZeroContents | false | Whether or not we zero chunk contents on |
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| | | allocation and deallocation. |
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+-----------------------------+---------+------------------------------------------------+
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Allocator related common Sanitizer options can also be passed through Scudo
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options, such as ``allocator_may_return_null``. A detailed list including those
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can be found here:
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https://github.com/google/sanitizers/wiki/SanitizerCommonFlags.
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