Disable vsyscall emulation
Requires the kernel to be built without CONFIG_LEGACY_VSYSCALL_EMULATE=y, favoring the stronger NONE or XONLY vsyscall choice instead.
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_LEGACY_VSYSCALL_EMULATE=y keeps a vsyscall page mapped at a fixed, well-known address and traps accesses to emulate the legacy calls. Although the mapping is non-executable, it still contains known, fixed-address contents that can serve as ROP gadgets in rare ASLR-bypass exploits. This is the weakest of the vsyscall choices; the rule prefers the stronger NONE (no mapping) or XONLY over EMULATE, so it flags a kernel built with emulation enabled.
What Pavois checks
Pavois reads the build configuration of the running kernel, /boot/config-$(uname -r) and the in-memory /proc/config.gz, and confirms CONFIG_LEGACY_VSYSCALL_EMULATE is not set to y. vsyscall mode is a compile-time choice, so reading the booted kernel's real config (not a stale /boot/config-*) shows which variant is actually active.
describe command("C=/boot/config-$(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_LEGACY_VSYSCALL_EMULATE=|PAVOIS_NO_KERNEL_CONFIG)'") do
its('stdout') { should_not match(/PAVOIS_NO_KERNEL_CONFIG/) }
its('stdout') { should_not match(/^CONFIG_LEGACY_VSYSCALL_EMULATE=y$/) }
endHow to verify it is applied
Run grep '^CONFIG_LEGACY_VSYSCALL' /boot/config-$(uname -r). Expected: CONFIG_LEGACY_VSYSCALL_EMULATE absent or is not set, with CONFIG_LEGACY_VSYSCALL_NONE=y (or _XONLY=y) selected instead.
Inspect & investigate
No dedicated log. The active vsyscall mode can be influenced at boot via the vsyscall= parameter in cat /proc/cmdline. Faults from blocked vsyscall access appear as segfaults in dmesg | grep -i -e vsyscall -e segfault or journalctl -k | grep -i vsyscall.
Remediation
No automated remediation: this is a kernel build-time choice, not a runtime tunable. Pavois's harden plan is empty. To fix it you must boot a kernel built with CONFIG_LEGACY_VSYSCALL_NONE=y (preferred) or _XONLY=y instead of emulate. At runtime you can harden a still-emulating kernel with the vsyscall=none boot parameter, but the compile-time choice this rule checks requires a different kernel.
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
Moving away from emulation toward NONE removes a compatibility shim for very old statically linked binaries (glibc older than ~2.14); modern software using the vDSO is unaffected. Audit for legacy static binaries first, emulation exists precisely as their fallback, so if one is required keep emulation or use vsyscall=emulate. Because the change requires a different kernel, keep the previous kernel entry in the bootloader.
Standards mapping
| Standard | Reference | Type | Version | Confidence |
|---|---|---|---|---|
| ANSSI BP-028 | R15 | 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.