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2 Scudo Hardened Allocator
3 ========================
12 The Scudo Hardened Allocator is a user-mode allocator based on LLVM Sanitizer's
13 CombinedAllocator, which aims at providing additional mitigations against heap
14 based vulnerabilities, while maintaining good performance.
16 Currently, the allocator supports (was tested on) the following architectures:
18 - i386 (& i686) (32-bit);
23 The name "Scudo" has been retained from the initial implementation (Escudo
24 meaning Shield in Spanish and Portuguese).
31 Scudo can be considered a Frontend to the Sanitizers' common allocator (later
32 referenced as the Backend). It is split between a Primary allocator, fast and
33 efficient, that services smaller allocation sizes, and a Secondary allocator
34 that services larger allocation sizes and is backed by the operating system
35 memory mapping primitives.
37 Scudo was designed with security in mind, but aims at striking a good balance
38 between security and performance. It is highly tunable and configurable.
42 Every chunk of heap memory will be preceded by a chunk header. This has two
43 purposes, the first one being to store various information about the chunk,
44 the second one being to detect potential heap overflows. In order to achieve
45 this, the header will be checksummed, involving the pointer to the chunk itself
46 and a global secret. Any corruption of the header will be detected when said
47 header is accessed, and the process terminated.
49 The following information is stored in the header:
51 - the 16-bit checksum;
52 - the class ID for that chunk, which is the "bucket" where the chunk resides
53 for Primary backed allocations, or 0 for Secondary backed allocations;
54 - the size (Primary) or unused bytes amount (Secondary) for that chunk, which is
55 necessary for computing the size of the chunk;
56 - the state of the chunk (available, allocated or quarantined);
57 - the allocation type (malloc, new, new[] or memalign), to detect potential
58 mismatches in the allocation APIs used;
59 - the offset of the chunk, which is the distance in bytes from the beginning of
60 the returned chunk to the beginning of the Backend allocation;
62 This header fits within 8 bytes, on all platforms supported.
64 The checksum is computed as a CRC32 (made faster with hardware support)
65 of the global secret, the chunk pointer itself, and the 8 bytes of header with
66 the checksum field zeroed out. It is not intended to be cryptographically
69 The header is atomically loaded and stored to prevent races. This is important
70 as two consecutive chunks could belong to different threads. We also want to
71 avoid any type of double fetches of information located in the header, and use
72 local copies of the header for this purpose.
76 A delayed freelist allows us to not return a chunk directly to the Backend, but
77 to keep it aside for a while. Once a criterion is met, the delayed freelist is
78 emptied, and the quarantined chunks are returned to the Backend. This helps
79 mitigate use-after-free vulnerabilities by reducing the determinism of the
80 allocation and deallocation patterns.
82 This feature is using the Sanitizer's Quarantine as its base, and the amount of
83 memory that it can hold is configurable by the user (see the Options section
88 It is important for the allocator to not make use of fixed addresses. We use
89 the dynamic base option for the SizeClassAllocator, allowing us to benefit
90 from the randomness of mmap.
97 The allocator static library can be built from the LLVM build tree thanks to
98 the ``scudo`` CMake rule. The associated tests can be exercised thanks to the
99 ``check-scudo`` CMake rule.
101 Linking the static library to your project can require the use of the
102 ``whole-archive`` linker flag (or equivalent), depending on your linker.
103 Additional flags might also be necessary.
105 Your linked binary should now make use of the Scudo allocation and deallocation
108 You may also build Scudo like this:
112 cd $LLVM/projects/compiler-rt/lib
113 clang++ -fPIC -std=c++11 -msse4.2 -O2 -I. scudo/*.cpp \
114 $(\ls sanitizer_common/*.{cc,S} | grep -v "sanitizer_termination\|sanitizer_common_nolibc") \
115 -shared -o scudo-allocator.so -pthread
117 and then use it with existing binaries as follows:
121 LD_PRELOAD=`pwd`/scudo-allocator.so ./a.out
125 With a recent version of Clang (post rL317337), the allocator can be linked with
126 a binary at compilation using the ``-fsanitize=scudo`` command-line argument, if
127 the target platform is supported. Currently, the only other Sanitizer Scudo is
128 compatible with is UBSan (eg: ``-fsanitize=scudo,undefined``). Compiling with
129 Scudo will also enforce PIE for the output binary.
133 Several aspects of the allocator can be configured through the following ways:
135 - by defining a ``__scudo_default_options`` function in one's program that
136 returns the options string to be parsed. Said function must have the following
137 prototype: ``extern "C" const char* __scudo_default_options(void)``.
139 - through the environment variable SCUDO_OPTIONS, containing the options string
140 to be parsed. Options defined this way will override any definition made
141 through ``__scudo_default_options``;
143 The options string follows a syntax similar to ASan, where distinct options
144 can be assigned in the same string, separated by colons.
146 For example, using the environment variable:
150 SCUDO_OPTIONS="DeleteSizeMismatch=1:QuarantineSizeKb=64" ./a.out
152 Or using the function:
156 extern "C" const char *__scudo_default_options() {
157 return "DeleteSizeMismatch=1:QuarantineSizeKb=64";
161 The following options are available:
163 +-----------------------------+----------------+----------------+------------------------------------------------+
164 | Option | 64-bit default | 32-bit default | Description |
165 +-----------------------------+----------------+----------------+------------------------------------------------+
166 | QuarantineSizeKb | 256 | 64 | The size (in Kb) of quarantine used to delay |
167 | | | | the actual deallocation of chunks. Lower value |
168 | | | | may reduce memory usage but decrease the |
169 | | | | effectiveness of the mitigation; a negative |
170 | | | | value will fallback to the defaults. |
171 +-----------------------------+----------------+----------------+------------------------------------------------+
172 | QuarantineChunksUpToSize | 2048 | 512 | Size (in bytes) up to which chunks can be |
173 | | | | quarantined. |
174 +-----------------------------+----------------+----------------+------------------------------------------------+
175 | ThreadLocalQuarantineSizeKb | 1024 | 256 | The size (in Kb) of per-thread cache use to |
176 | | | | offload the global quarantine. Lower value may |
177 | | | | reduce memory usage but might increase |
178 | | | | contention on the global quarantine. |
179 +-----------------------------+----------------+----------------+------------------------------------------------+
180 | DeallocationTypeMismatch | true | true | Whether or not we report errors on |
181 | | | | malloc/delete, new/free, new/delete[], etc. |
182 +-----------------------------+----------------+----------------+------------------------------------------------+
183 | DeleteSizeMismatch | true | true | Whether or not we report errors on mismatch |
184 | | | | between sizes of new and delete. |
185 +-----------------------------+----------------+----------------+------------------------------------------------+
186 | ZeroContents | false | false | Whether or not we zero chunk contents on |
187 | | | | allocation and deallocation. |
188 +-----------------------------+----------------+----------------+------------------------------------------------+
190 Allocator related common Sanitizer options can also be passed through Scudo
191 options, such as ``allocator_may_return_null``. A detailed list including those
193 https://github.com/google/sanitizers/wiki/SanitizerCommonFlags.