Re: [PATCH v2] x86/bugs: protect against userspace-userspace spectreRSB

[Tim Chen]

On 07/26/2018 04:14 AM, Jiri Kosina wrote:
> From: Jiri Kosina <jkosina@xxxxxxx>
> 
> The article "Spectre Returns! Speculation Attacks using the Return Stack 
> Buffer" [1] describes two new (sub-)variants of spectrev2-like attack, 
> making use solely of the RSB contents even on CPUs that don't fallback to 
> BTB on RSB underflow (Skylake+).
> 
> Mitigate userspace-userspace attacks by always unconditionally filling RSB on
> context switch when generic spectrev2 mitigation has been enabled.
> 
> [1] https://arxiv.org/pdf/1807.07940.pdf
> 
> Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
> ---
> 
> v1 -> v2: 
> 
> 	- Fixed typos/capatalization in SpectreRSB name
> 	- Josh's Reviewed-by
> 
>  arch/x86/kernel/cpu/bugs.c | 38 +++++++-------------------------------
>  1 file changed, 7 insertions(+), 31 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 5c0ea39311fe..bc8c43b22460 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -313,23 +313,6 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
>  	return cmd;
>  }
>  
> -/* Check for Skylake-like CPUs (for RSB handling) */
> -static bool __init is_skylake_era(void)
> -{
> -	if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL &&
> -	    boot_cpu_data.x86 == 6) {
> -		switch (boot_cpu_data.x86_model) {
> -		case INTEL_FAM6_SKYLAKE_MOBILE:
> -		case INTEL_FAM6_SKYLAKE_DESKTOP:
> -		case INTEL_FAM6_SKYLAKE_X:
> -		case INTEL_FAM6_KABYLAKE_MOBILE:
> -		case INTEL_FAM6_KABYLAKE_DESKTOP:
> -			return true;
> -		}
> -	}
> -	return false;
> -}
> -
>  static void __init spectre_v2_select_mitigation(void)
>  {
>  	enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
> @@ -390,22 +373,15 @@ static void __init spectre_v2_select_mitigation(void)
>  	pr_info("%s\n", spectre_v2_strings[mode]);
>  
>  	/*
> -	 * If neither SMEP nor PTI are available, there is a risk of
> -	 * hitting userspace addresses in the RSB after a context switch
> -	 * from a shallow call stack to a deeper one. To prevent this fill
> -	 * the entire RSB, even when using IBRS.
> +	 * If spectre v2 protection has been enabled, unconditionally fill
> +	 * RSB during a context switch; this protects against two independent
> +	 * issues:
>  	 *
> -	 * Skylake era CPUs have a separate issue with *underflow* of the
> -	 * RSB, when they will predict 'ret' targets from the generic BTB.
> -	 * The proper mitigation for this is IBRS. If IBRS is not supported
> -	 * or deactivated in favour of retpolines the RSB fill on context
> -	 * switch is required.
> +	 *	- RSB underflow (and switch to BTB) on Skylake+
> +	 *	- SpectreRSB variant of spectre v2 on X86_BUG_SPECTRE_V2 CPUs
>  	 */
> -	if ((!boot_cpu_has(X86_FEATURE_PTI) &&
> -	     !boot_cpu_has(X86_FEATURE_SMEP)) || is_skylake_era()) {
> -		setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
> -		pr_info("Spectre v2 mitigation: Filling RSB on context switch\n");
> -	}
> +	setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
> +	pr_info("Spectre v2 / SpectreRSB mitigation: Filling RSB on context switch\n");
>  
>  	/* Initialize Indirect Branch Prediction Barrier if supported */
>  	if (boot_cpu_has(X86_FEATURE_IBPB)) {
> 

Thanks for the patch.  Looks good.  

Acked-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>

Tim