From: Sean Christopherson <seanjc@xxxxxxxxxx> [ Upstream commit ce0abef6a1d540acef85068e0e82bdf1fbeeb0e9 ] Explicitly disallow enabling mitigations at runtime for kernels that were built with CONFIG_CPU_MITIGATIONS=n, as some architectures may omit code entirely if mitigations are disabled at compile time. E.g. on x86, a large pile of Kconfigs are buried behind CPU_MITIGATIONS, and trying to provide sane behavior for retroactively enabling mitigations is extremely difficult, bordering on impossible. E.g. page table isolation and call depth tracking require build-time support, BHI mitigations will still be off without additional kernel parameters, etc. [ bp: Touchups. ] Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx> Acked-by: Borislav Petkov (AMD) <bp@xxxxxxxxx> Link: https://lore.kernel.org/r/20240420000556.2645001-3-seanjc@xxxxxxxxxx Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- Documentation/admin-guide/kernel-parameters.txt | 3 +++ arch/x86/Kconfig | 8 ++++++-- include/linux/cpu.h | 11 +++++++++++ kernel/cpu.c | 14 ++++++++++---- 4 files changed, 30 insertions(+), 6 deletions(-) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 31fdaf4fe9dd8..9bfc972af2403 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -3399,6 +3399,9 @@ arch-independent options, each of which is an aggregation of existing arch-specific options. + Note, "mitigations" is supported if and only if the + kernel was built with CPU_MITIGATIONS=y. + off Disable all optional CPU mitigations. This improves system performance, but it may also diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index 6f49999a6b838..bfccf1213871b 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -2474,9 +2474,13 @@ menuconfig CPU_MITIGATIONS help Say Y here to enable options which enable mitigations for hardware vulnerabilities (usually related to speculative execution). + Mitigations can be disabled or restricted to SMT systems at runtime + via the "mitigations" kernel parameter. - If you say N, all mitigations will be disabled. You really - should know what you are doing to say so. + If you say N, all mitigations will be disabled. This CANNOT be + overridden at runtime. + + Say 'Y', unless you really know what you are doing. if CPU_MITIGATIONS diff --git a/include/linux/cpu.h b/include/linux/cpu.h index 8654714421a0d..75f0344bd3b94 100644 --- a/include/linux/cpu.h +++ b/include/linux/cpu.h @@ -219,7 +219,18 @@ void cpuhp_report_idle_dead(void); static inline void cpuhp_report_idle_dead(void) { } #endif /* #ifdef CONFIG_HOTPLUG_CPU */ +#ifdef CONFIG_CPU_MITIGATIONS extern bool cpu_mitigations_off(void); extern bool cpu_mitigations_auto_nosmt(void); +#else +static inline bool cpu_mitigations_off(void) +{ + return true; +} +static inline bool cpu_mitigations_auto_nosmt(void) +{ + return false; +} +#endif #endif /* _LINUX_CPU_H_ */ diff --git a/kernel/cpu.c b/kernel/cpu.c index f8a0406ce8ba5..bac70ea54e349 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -3196,6 +3196,7 @@ void __init boot_cpu_hotplug_init(void) this_cpu_write(cpuhp_state.target, CPUHP_ONLINE); } +#ifdef CONFIG_CPU_MITIGATIONS /* * These are used for a global "mitigations=" cmdline option for toggling * optional CPU mitigations. @@ -3206,9 +3207,7 @@ enum cpu_mitigations { CPU_MITIGATIONS_AUTO_NOSMT, }; -static enum cpu_mitigations cpu_mitigations __ro_after_init = - IS_ENABLED(CONFIG_CPU_MITIGATIONS) ? CPU_MITIGATIONS_AUTO : - CPU_MITIGATIONS_OFF; +static enum cpu_mitigations cpu_mitigations __ro_after_init = CPU_MITIGATIONS_AUTO; static int __init mitigations_parse_cmdline(char *arg) { @@ -3224,7 +3223,6 @@ static int __init mitigations_parse_cmdline(char *arg) return 0; } -early_param("mitigations", mitigations_parse_cmdline); /* mitigations=off */ bool cpu_mitigations_off(void) @@ -3239,3 +3237,11 @@ bool cpu_mitigations_auto_nosmt(void) return cpu_mitigations == CPU_MITIGATIONS_AUTO_NOSMT; } EXPORT_SYMBOL_GPL(cpu_mitigations_auto_nosmt); +#else +static int __init mitigations_parse_cmdline(char *arg) +{ + pr_crit("Kernel compiled without mitigations, ignoring 'mitigations'; system may still be vulnerable\n"); + return 0; +} +#endif +early_param("mitigations", mitigations_parse_cmdline); -- 2.43.0
