Address Space Layout Randomization (ASLR) is an exploit mitigation technique implemented in the majority of modern operating systems. It involves randomly positioning the base address of an executable and the position of libraries, heap, and stack, in a process’s address space. Although over the years ASLR proved to not guarantee full OS security on its own, this mechanism can make exploitation more difficult.
Tests on the tier 1 64-bit architectures demonstrated that the ASLR is stable and does not result in noticeable performance degradation, therefore it should be safe to enable this mechanism by default. Moreover its effectiveness is increased for PIE (Position Independent Executable) binaries. Thanks to commit 9a227a2fd642 (“Enable PIE by default on 64-bit architectures”), building from src is not necessary to have PIE binaries. It is enough to control usage of ASLR in the OS solely by setting the appropriate sysctls.
This patch toggles the kernel settings to use address map randomization for PIE & non-PIE 64-bit binaries. It also disables SBRK, in order to allow utilization of the bss grow region for mappings. The latter has no effect if ASLR is disabled, so apply it to all architectures.
As for the drawbacks, a consequence of using the ASLR is more significant VM fragmentation, hence the issues may be encountered in the systems with a limited address space in high memory consumption cases, such as buildworld. As a result, although the tests on 32-bit architectures with ASLR enabled were mostly on par with what was observed on 64-bit ones, the defaults for the
former are not changed at this time. Also, for the sake of safety keep the feature disabled for 32-bit executables on 64-bit machines, too.
The committed change affects the overall OS operation, so the following should be taken into consideration:
- Address space fragmentation.
- A changed ABI due to modified layout of address space.
- More complicated debugging due to:
- Non-reproducible address space layout between runs.
- Some debuggers automatically disable ASLR for spawned processes, making target’s environment different between debug and non-debug runs.
In order to confirm/rule-out the dependency of any encountered issue on ASLR it is strongly advised to re-run the test with the feature disabled - it can be done by setting the following sysctls in the /etc/sysctl.conf file: