mirror of
https://github.com/RPCSX/llvm.git
synced 2024-12-26 14:15:53 +00:00
1e3f45d363
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278180 91177308-0d34-0410-b5e6-96231b3b80d8
168 lines
7.0 KiB
ReStructuredText
168 lines
7.0 KiB
ReStructuredText
========================
|
|
Scudo Hardened Allocator
|
|
========================
|
|
|
|
.. contents::
|
|
:local:
|
|
:depth: 1
|
|
|
|
Introduction
|
|
============
|
|
|
|
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).
|
|
|
|
Design
|
|
======
|
|
|
|
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
|
|
below).
|
|
|
|
Randomness
|
|
----------
|
|
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.
|
|
|
|
Usage
|
|
=====
|
|
|
|
Library
|
|
-------
|
|
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
|
|
functions.
|
|
|
|
You may also build Scudo like this:
|
|
|
|
.. code::
|
|
|
|
cd $LLVM/projects/compiler-rt/lib
|
|
clang++ -fPIC -std=c++11 -msse4.2 -mcx16 -O2 -I. scudo/*.cpp \
|
|
$(\ls sanitizer_common/*.{cc,S} | grep -v "sanitizer_termination\|sanitizer_common_nolibc") \
|
|
-shared -o scudo-allocator.so -lpthread
|
|
|
|
and then use it with existing binaries as follows:
|
|
|
|
.. code::
|
|
|
|
LD_PRELOAD=`pwd`/scudo-allocator.so ./a.out
|
|
|
|
Options
|
|
-------
|
|
Several aspects of the allocator can be configured through the following ways:
|
|
|
|
- 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()``.
|
|
|
|
- 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``;
|
|
|
|
The options string 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:
|
|
|
|
.. code::
|
|
|
|
SCUDO_OPTIONS="DeleteSizeMismatch=1:QuarantineSizeMb=16" ./a.out
|
|
|
|
Or using the function:
|
|
|
|
.. code::
|
|
|
|
extern "C" const char *__scudo_default_options() {
|
|
return "DeleteSizeMismatch=1:QuarantineSizeMb=16";
|
|
}
|
|
|
|
|
|
The following options are available:
|
|
|
|
+-----------------------------+---------+------------------------------------------------+
|
|
| Option | Default | Description |
|
|
+-----------------------------+---------+------------------------------------------------+
|
|
| QuarantineSizeMb | 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 | 1024 | 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. |
|
|
+-----------------------------+---------+------------------------------------------------+
|
|
| DeallocationTypeMismatch | true | Whether or not we report errors on |
|
|
| | | malloc/delete, new/free, new/delete[], etc. |
|
|
+-----------------------------+---------+------------------------------------------------+
|
|
| DeleteSizeMismatch | true | Whether or not we report errors on mismatch |
|
|
| | | between sizes of new and delete. |
|
|
+-----------------------------+---------+------------------------------------------------+
|
|
| ZeroContents | false | Whether or not we zero chunk contents on |
|
|
| | | allocation and deallocation. |
|
|
+-----------------------------+---------+------------------------------------------------+
|
|
|
|
Allocator related common Sanitizer options can also be passed through Scudo
|
|
options, such as ``allocator_may_return_null``. A detailed list including those
|
|
can be found here:
|
|
https://github.com/google/sanitizers/wiki/SanitizerCommonFlags.
|
|
|