[llvm] r271968 - [sanitizer] Initial implementation of a Hardened Allocator

Kostya Serebryany via llvm-commits llvm-commits at lists.llvm.org
Mon Jun 6 18:20:28 PDT 2016

Author: kcc
Date: Mon Jun  6 20:20:26 2016
New Revision: 271968

URL: http://llvm.org/viewvc/llvm-project?rev=271968&view=rev
[sanitizer] Initial implementation of a Hardened Allocator

This is an initial implementation of a Hardened Allocator based on Sanitizer Common's CombinedAllocator.
It aims at mitigating heap based vulnerabilities by adding several features to the base allocator, while staying relatively fast.
The following were implemented:
- additional consistency checks on the allocation function parameters and on the heap chunks;
- use of checksum protected chunk header, to detect corruption;
- randomness to the allocator base;
- delayed freelist (quarantine), to mitigate use after free and overall determinism.
Additional mitigations are in the works.

Reviewers: eugenis, aizatsky, pcc, krasin, vitalybuka, glider, dvyukov, kcc

Subscribers: kubabrecka, filcab, llvm-commits

Differential Revision: http://reviews.llvm.org/D20084


Added: llvm/trunk/docs/ScudoHardenedAllocator.rst
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/ScudoHardenedAllocator.rst?rev=271968&view=auto
--- llvm/trunk/docs/ScudoHardenedAllocator.rst (added)
+++ llvm/trunk/docs/ScudoHardenedAllocator.rst Mon Jun  6 20:20:26 2016
@@ -0,0 +1,117 @@
+Scudo Hardened Allocator
+.. contents::
+   :local:
+   :depth: 1
+The Scudo Hardened Allocator is a user-mode allocator based on LLVM Sanitizer's
+CombinedAllocator, which aims at providing additional mitigations against heap
+based vulnerabilities, while maintaining good performance.
+The name "Scudo" has been retained from the initial implementation (Escudo
+meaning Shield in Spanish and Portuguese).
+Chunk Header
+Every chunk of heap memory will be preceded by a chunk header. This has two
+purposes, the first one being to store various information about the chunk,
+the second one being to detect potential heap overflows. In order to achieve
+this, the header will be checksumed, involving the pointer to the chunk itself
+and a global secret. Any corruption of the header will be detected when said
+header is accessed, and the process terminated.
+The following information is stored in the header:
+- the 16-bit checksum;
+- the user requested size for that chunk, which is necessary for reallocation
+  purposes;
+- 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;
+- whether or not the chunk is offseted (ie: if the chunk beginning is different
+  than the backend allocation beginning, which is most often the case with some
+  aligned allocations);
+- the associated offset;
+- a 16-bit salt.
+On x64, which is currently the only architecture supported, the header fits
+within 16-bytes, which works nicely with the minimum alignment requirements.
+The checksum is computed as a CRC32 (requiring the SSE 4.2 instruction set)
+of the global secret, the chunk pointer itself, and the 16 bytes of header with
+the checksum field zeroed out.
+The header is atomically loaded and stored to prevent races (this requires
+platform support such as the cmpxchg16b instruction). This is important as two
+consecutive chunks could belong to different threads. We also want to avoid
+any type of double fetches of information located in the header, and use local
+copies of the header for this purpose.
+Delayed Freelist
+A delayed freelist allows us to not return a chunk directly to the backend, but
+to keep it aside for a while. Once a criterion is met, the delayed freelist is
+emptied, and the quarantined chunks are returned to the backend. This helps
+mitigate use-after-free vulnerabilities by reducing the determinism of the
+allocation and deallocation patterns.
+This feature is using the Sanitizer's Quarantine as its base, and the amount of
+memory that it can hold is configurable by the user (see the Options section
+It is important for the allocator to not make use of fixed addresses. We use
+the dynamic base option for the SizeClassAllocator, allowing us to benefit
+from the randomness of mmap.
+The allocator static library can be built from the LLVM build tree thanks to
+the "scudo" CMake rule. The associated tests can be exercised thanks to the
+"check-scudo" 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
+Several aspects of the allocator can be configured through environment options,
+following the usual ASan options syntax, through the variable SCUDO_OPTIONS.
+For example: SCUDO_OPTIONS="DeleteSizeMismatch=1:QuarantineSizeMb=16".
+The following options are available:
+- QuarantineSizeMb (integer, defaults to 64): the size (in Mb) 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 a default of 64Mb;
+- ThreadLocalQuarantineSizeKb (integer, default to 1024): the size (in Kb) of
+  per-thread cache used to offload the global quarantine. Lower value may
+  reduce memory usage but might increase the contention on the global
+  quarantine.
+- DeallocationTypeMismatch (boolean, defaults to true): whether or not we report
+  errors on malloc/delete, new/free, new/delete[], etc;
+- DeleteSizeMismatch (boolean, defaults to true): whether or not we report
+  errors on mismatch between size of new and delete;
+- ZeroContents (boolean, defaults to false): whether or not we zero chunk
+  contents on allocation and deallocation.

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