Re: [RFC PATCH 07/56] x86/bugs: Reset spectre_v2_user mitigations

Brendan Jackman
2025-10-16T12:54:06Z

On Mon Oct 13, 2025 at 2:33 PM UTC, David Kaplan wrote:
> Add function to reset spectre_v2_user mitigations back to their boot-time
> defaults.
>
> Signed-off-by: David Kaplan <david.kaplan@amd.com>
> ---
> arch/x86/kernel/cpu/bugs.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 1f56ccb5f641..4ca46f58e384 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -2056,6 +2056,18 @@ static void __init spectre_v2_user_apply_mitigation(void)
> }
> }
>
> +#ifdef CONFIG_DYNAMIC_MITIGATIONS
> +static void spectre_v2_user_reset_mitigation(void)
> +{
> + static_branch_disable(&switch_vcpu_ibpb);
> + static_branch_disable(&switch_mm_always_ibpb);
> + static_branch_disable(&switch_mm_cond_ibpb);
> + spectre_v2_user_stibp = SPECTRE_V2_USER_NONE;
> + spectre_v2_user_ibpb = SPECTRE_V2_USER_NONE;
> + spectre_v2_user_cmd = SPECTRE_V2_USER_CMD_AUTO;
> +}
> +#endif
> +
> static const char * const spectre_v2_strings[] = {
> [SPECTRE_V2_NONE] = "Vulnerable",
> [SPECTRE_V2_RETPOLINE] = "Mitigation: Retpolines",
> @@ -3844,5 +3856,6 @@ void arch_cpu_reset_mitigations(void)
> spectre_v1_reset_mitigation();
> spectre_v2_reset_mitigation();
> retbleed_reset_mitigation();
> + spectre_v2_user_reset_mitigation();
> }
> #endif

I think this might be failing to account for task state? E.g. if a
user boots with spectre_v2=off then we ignore the PR_SPEC_DISABLE calls
that would enable IBPB-on-context-switch for that task. Then if they
enable it via this dynamic interface they probably expect their
PR_SPEC_DISABLE to take effect retroactively. I don't think it will with
the current code, do I have that right?