Add _FORTIFY_SOURCE=3 to distribution build flags
This document represents a proposed Change. As part of the Changes process, proposals are publicly announced in order to receive community feedback. This proposal will only be implemented if approved by the Fedora Engineering Steering Committee.
Summary
Replace the current _FORTIFY_SOURCE=2 with _FORTIFY_SOURCE=3 to improve mitigation of security issues arising from buffer overflows in packages in Fedora.
Owner
- Name: Siddhesh Poyarekar
- Email: sipoyare@redhat.com
Current status
- Targeted release: Fedora Linux 38
- Last updated: 2022-12-06
- devel thread
- FESCo issue: <will be assigned by the Wrangler>
- Tracker bug: <will be assigned by the Wrangler>
- Release notes issue: <will be assigned by the Wrangler>
Detailed Description
Default C and C++ compiler flags to build packages in Fedora currently includes -Wp,-D_FORTIFY_SOURCE=2, which enables fortification of some functions in glibc, thus providing some mitigation against buffer overflows. Since glibc 2.34 and GCC 12, there has been a new fortification level (_FORTIFY_SOURCE=3) which improves the coverage of this mitigation.
The core change to bring in this mitigation is to change the default build flags in redhat-rpm-config so that packages build by default with -Wp,-D_FORTIFY_SOURCE=3. There are packages (e.g. systemd) that do not interact well with _FORTIFY_SOURCE and will also need a workaround to downgrade fortification to level 2. The change will also include this override.
Benefit to Fedora
Analysis of packages in Fedora rawhide indicate that the improvement of mitigation coverage is on average over 2.4x, in some cases protecting more than half of the fortified glibc calls in the target application.
This change will thus harden Fedora to a significant extent, thus making it a more secure distribution out of the box.
Scope
- Proposal owners:
Post a merge request to redhat-rpm-config with the actual change to build flags.
- Other developers:
Resolve bugs filed for build failures, either by fixing the bug exposed by _FORTIFY_SOURCE=3 or by disabling _FORTIFY_SOURCE=3 for the package if it is a false positive or if the package is unable to adapt to the change.
- Release engineering: Mass rebuild required
- Policies and guidelines: Guidelines should include workaround for packages that fail to build with
-Wp,-D_FORTIFY_SOURCE=3due to a false positive.
- Trademark approval: N/A (not needed for this Change)
Upgrade/compatibility impact
No ABI change, so there should be no impact on compatibility in a mixed environment.
How To Test
- Smoke testing of packages to ensure that they continue to work correctly. Some packages may have overflows exposed at runtime, which may need to be fixed.
User Experience
No user-visible functional change.
Performance note
Given that _FORTIFY_SOURCE=3 generates size expressions instead of constants as in _FORTIFY_SOURCE=2 and also succeeds in fortification in more cases than _FORTIFY_SOURCE=2, there is a theoretical concern of a performance overhead due to this. This concern is limited to cases where the fortified function is in the hot path and at the same time, the size expression needs to be evaluated separately in each iteration of the hot path.
Size expressions are already available to the compiler in the function and the worst case impact in that context is some amount of register pressure in that function. As a result, the performance overhead is not expected to be too high or far-reaching. Per-application performance benchmarks may be useful in understanding the impact, but the impact is not expected to be broad.
In practice, no performance issues have been reported in distributions that have already enabled _FORTIFY_SOURCE=3 by default. The linux kernel will improve its fortification to match _FORTIFY_SOURCE=3 by default in a future release.
Dependencies
None.
Contingency Plan
- Contingency mechanism: (What to do? Who will do it?) If too many packages are found to be broken at runtime, the default for fortification will be left at
_FORTIFY_SOURCE=2for Fedora 38. Change owner will do this inredhat-rpm-config - Contingency deadline: Beta freeze
- Blocks release? Yes
- Blocks product? No
Documentation
More context on _FORTIFY_SOURCE=3 improvements.