CopperheadOS disables the kernel's CONFIG_AIO feature

It isn't used or exposed by the base system and is a dubious feature.
It performs no better than thread pools and it can still block, along
with having coverage of only a tiny portion of blocking system calls
even when considering only commonly used system calls for IO.
There are no known compatibility issues caused by having this disabled.
Since this is such a dubious niche feature, it's also very poorly tested
and it doesn't get much attention. Proposed improvements have been blocked
based on the concern that POSIX AIO is such a bad interface that trying
to improve/extend it would be harmful. Following the lead of CopperheadOS
on this front has been proposed and accepted upstream for the recommended
Android kernel configuration used to derive device specific configurations.

https://github.com/AndroidHardeningArchive/documentation/blob/master/technical_overview.md#attack-surface-reduction

diff --git a/kconfig-hardened-check.py b/kconfig-hardened-check.py
index 197d82ed33d8c2db2ac0ddd09d09cb2be3970b6c..c5dcb06a1b67f9f8f1e292f14aaa8ca82aae9db0 100755 (executable)
--- a/kconfig-hardened-check.py
+++ b/kconfig-hardened-check.py
@@ -426,6 +426,8 @@ def construct_checklist(checklist, arch):
     checklist.append(AND(OptCheck('LDISC_AUTOLOAD',           'is not set', 'clipos', 'cut_attack_surface'), \
                          VerCheck((5, 1)))) # LDISC_AUTOLOAD can be disabled since v5.1
 
+    checklist.append(OptCheck('AIO',                  'is not set', 'copperhead', 'cut_attack_surface'))
+
     checklist.append(OptCheck('MMIOTRACE',            'is not set', 'my', 'cut_attack_surface')) # refers to LOCKDOWN (permissive)
     checklist.append(OptCheck('LIVEPATCH',            'is not set', 'my', 'cut_attack_surface'))
     checklist.append(OptCheck('IP_DCCP',              'is not set', 'my', 'cut_attack_surface'))