1 ======================== 2 Scudo Hardened Allocator 3 ======================== 4 5 .. contents:: 6 :local: 7 :depth: 1 8 9 Introduction 10 ============ 11 The Scudo Hardened Allocator is a user-mode allocator based on LLVM Sanitizer's 12 CombinedAllocator, which aims at providing additional mitigations against heap 13 based vulnerabilities, while maintaining good performance. 14 15 The name "Scudo" has been retained from the initial implementation (Escudo 16 meaning Shield in Spanish and Portuguese). 17 18 Design 19 ====== 20 Chunk Header 21 ------------ 22 Every chunk of heap memory will be preceded by a chunk header. This has two 23 purposes, the first one being to store various information about the chunk, 24 the second one being to detect potential heap overflows. In order to achieve 25 this, the header will be checksumed, involving the pointer to the chunk itself 26 and a global secret. Any corruption of the header will be detected when said 27 header is accessed, and the process terminated. 28 29 The following information is stored in the header: 30 31 - the 16-bit checksum; 32 - the user requested size for that chunk, which is necessary for reallocation 33 purposes; 34 - the state of the chunk (available, allocated or quarantined); 35 - the allocation type (malloc, new, new[] or memalign), to detect potential 36 mismatches in the allocation APIs used; 37 - whether or not the chunk is offseted (ie: if the chunk beginning is different 38 than the backend allocation beginning, which is most often the case with some 39 aligned allocations); 40 - the associated offset; 41 - a 16-bit salt. 42 43 On x64, which is currently the only architecture supported, the header fits 44 within 16-bytes, which works nicely with the minimum alignment requirements. 45 46 The checksum is computed as a CRC32 (requiring the SSE 4.2 instruction set) 47 of the global secret, the chunk pointer itself, and the 16 bytes of header with 48 the checksum field zeroed out. 49 50 The header is atomically loaded and stored to prevent races (this requires 51 platform support such as the cmpxchg16b instruction). This is important as two 52 consecutive chunks could belong to different threads. We also want to avoid 53 any type of double fetches of information located in the header, and use local 54 copies of the header for this purpose. 55 56 Delayed Freelist 57 ----------------- 58 A delayed freelist allows us to not return a chunk directly to the backend, but 59 to keep it aside for a while. Once a criterion is met, the delayed freelist is 60 emptied, and the quarantined chunks are returned to the backend. This helps 61 mitigate use-after-free vulnerabilities by reducing the determinism of the 62 allocation and deallocation patterns. 63 64 This feature is using the Sanitizer's Quarantine as its base, and the amount of 65 memory that it can hold is configurable by the user (see the Options section 66 below). 67 68 Randomness 69 ---------- 70 It is important for the allocator to not make use of fixed addresses. We use 71 the dynamic base option for the SizeClassAllocator, allowing us to benefit 72 from the randomness of mmap. 73 74 Usage 75 ===== 76 77 Library 78 ------- 79 The allocator static library can be built from the LLVM build tree thanks to 80 the "scudo" CMake rule. The associated tests can be exercised thanks to the 81 "check-scudo" CMake rule. 82 83 Linking the static library to your project can require the use of the 84 "whole-archive" linker flag (or equivalent), depending on your linker. 85 Additional flags might also be necessary. 86 87 Your linked binary should now make use of the Scudo allocation and deallocation 88 functions. 89 90 Options 91 ------- 92 Several aspects of the allocator can be configured through environment options, 93 following the usual ASan options syntax, through the variable SCUDO_OPTIONS. 94 95 For example: SCUDO_OPTIONS="DeleteSizeMismatch=1:QuarantineSizeMb=16". 96 97 The following options are available: 98 99 - QuarantineSizeMb (integer, defaults to 64): the size (in Mb) of quarantine 100 used to delay the actual deallocation of chunks. Lower value may reduce 101 memory usage but decrease the effectiveness of the mitigation; a negative 102 value will fallback to a default of 64Mb; 103 104 - ThreadLocalQuarantineSizeKb (integer, default to 1024): the size (in Kb) of 105 per-thread cache used to offload the global quarantine. Lower value may 106 reduce memory usage but might increase the contention on the global 107 quarantine. 108 109 - DeallocationTypeMismatch (boolean, defaults to true): whether or not we report 110 errors on malloc/delete, new/free, new/delete[], etc; 111 112 - DeleteSizeMismatch (boolean, defaults to true): whether or not we report 113 errors on mismatch between size of new and delete; 114 115 - ZeroContents (boolean, defaults to false): whether or not we zero chunk 116 contents on allocation and deallocation. 117 118