Re: [PATCH v2] x86/bugs: Warn when "ibrs" mitigation is selected on Enhanced IBRS parts

[Thadeu Lima de Souza Cascardo <cascardo@canonical.com>]

On Thu, Jul 14, 2022 at 04:15:35PM -0700, Pawan Gupta wrote:
> IBRS mitigation for spectre_v2 forces write to MSR_IA32_SPEC_CTRL at
> every kernel entry/exit. On Enhanced IBRS parts setting
> MSR_IA32_SPEC_CTRL[IBRS] only once at boot is sufficient. MSR writes at
> every kernel entry/exit incur unnecessary performance loss.
> 
> When Enhanced IBRS feature is present, print a warning about this
> unnecessary performance loss.
> 
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>

Reviewed-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

> ---
> v1->v2: Instead of changing the mitigation, print a warning about the
>         perf loss.
> 
> v1: https://lore.kernel.org/lkml/0456b35fb9ef957d9a9138e0913fb1a3fd445dff.1657747493.git.pawan.kumar.gupta@linux.intel.com/
> 
>  arch/x86/kernel/cpu/bugs.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 0dd04713434b..1c54fad3c54b 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -975,6 +975,7 @@ static inline const char *spectre_v2_module_string(void) { return ""; }
>  #define SPECTRE_V2_LFENCE_MSG "WARNING: LFENCE mitigation is not recommended for this CPU, data leaks possible!\n"
>  #define SPECTRE_V2_EIBRS_EBPF_MSG "WARNING: Unprivileged eBPF is enabled with eIBRS on, data leaks possible via Spectre v2 BHB attacks!\n"
>  #define SPECTRE_V2_EIBRS_LFENCE_EBPF_SMT_MSG "WARNING: Unprivileged eBPF is enabled with eIBRS+LFENCE mitigation and SMT, data leaks possible via Spectre v2 BHB attacks!\n"
> +#define SPECTRE_V2_IBRS_PERF_MSG "WARNING: IBRS mitigation selected on Enhanced IBRS CPU, this may cause unnecessary performance loss\n"
>  
>  #ifdef CONFIG_BPF_SYSCALL
>  void unpriv_ebpf_notify(int new_state)
> @@ -1415,6 +1416,8 @@ static void __init spectre_v2_select_mitigation(void)
>  
>  	case SPECTRE_V2_IBRS:
>  		setup_force_cpu_cap(X86_FEATURE_KERNEL_IBRS);
> +		if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED))
> +			pr_warn(SPECTRE_V2_IBRS_PERF_MSG);
>  		break;
>  
>  	case SPECTRE_V2_LFENCE:
> 
> base-commit: 4a57a8400075bc5287c5c877702c68aeae2a033d
> -- 
> 2.35.3
> 
>