Randomize slab freelist
Requires the kernel to be built with CONFIG_SLAB_FREELIST_RANDOM=y, randomizing slab object allocation order.
Checked against the resolved running state (e.g. sshd -T, sysctl, systemctl show), catches drop-ins and Includes a file read would miss. Caveat: runtime ≠ persistence; a value correct now may not survive a reboot.
Pavois asserts the effective configuration, the live, resolved state, not a file. File-based scanners (OVAL/SCAP, Lynis) miss Includes, drop-ins and runtime defaults; this check sees what is actually applied.
A mapping is a cross-reference to where each standard places this requirement, anchored and cross-validated, not a claim of equivalence. A passing check is evidence toward these references, how to read it.
Why this rule matters
CONFIG_SLAB_FREELIST_RANDOM=y randomizes the order in which free objects are handed out within a slab cache. This breaks the predictable, sequential layout that heap-overflow exploits rely on to place an attacker-controlled object adjacent to a target object, making reliable heap grooming much harder.
What Pavois checks
Pavois reads the build configuration of the running kernel via /boot/config-$(uname -r) or /proc/config.gz and confirms CONFIG_SLAB_FREELIST_RANDOM=y. Inspecting the effective config of the booted kernel, not a default or packaged file, proves the protection is compiled into the kernel actually in use.
describe command("C=/boot/config-$(uname -r); if [ -r \"$C\" ]; then cat \"$C\"; elif zcat /proc/config.gz 2>/dev/null | head -1 | grep -q .; then zcat /proc/config.gz; else echo PAVOIS_NO_KERNEL_CONFIG; fi | grep -E '^(CONFIG_SLAB_FREELIST_RANDOM=|PAVOIS_NO_KERNEL_CONFIG)'") do
its('stdout') { should_not match(/PAVOIS_NO_KERNEL_CONFIG/) }
its('stdout') { should match(/^CONFIG_SLAB_FREELIST_RANDOM=y$/) }
endHow to verify it is applied
grep '^CONFIG_SLAB_FREELIST_RANDOM=' /boot/config-$(uname -r) || zcat /proc/config.gz | grep '^CONFIG_SLAB_FREELIST_RANDOM='
Expected output: CONFIG_SLAB_FREELIST_RANDOM=y.
Inspect & investigate
This is a static build-time mitigation with no runtime log output. Verify it only with the grep command above; there is no event to observe in system logs.
Remediation
No automated remediation is shipped: CONFIG_SLAB_FREELIST_RANDOM is a compile-time kernel option and cannot be enabled at runtime. The fix is to boot a kernel built with it enabled, the stock distribution kernels for the supported OSes already do. For a custom kernel, set CONFIG_SLAB_FREELIST_RANDOM=y and rebuild.
Pavois applies this with its own harden engine, the plan below, not a shell script:
| resource | kernel_build |
|---|
pavois harden plan localwhere the target is local, a user@host SSH alias, or a container , Docs
Impact & precautions
Without freelist randomization, slab allocation order is deterministic, giving heap-overflow exploits a reliable way to position objects and improving their success rate. Precautions: remediation means changing kernels and rebooting, schedule a maintenance window and keep the prior kernel in GRUB. The performance impact is negligible for typical workloads.
Standards mapping
| Standard | Reference | Type | Version | Confidence |
|---|---|---|---|---|
| ANSSI BP-028 | R17 | direct | 2.0 | high |
Each reference is a cross-reference anchored in the upstream benchmark and cross-validated against the SCAP Security Guide and ansible-lockdown, not a claim of equivalence. Direct = a prescriptive, line-level requirement; supporting = an abstract control family (NIST) the check provides evidence toward. How to read a mapping.