Re: [PATCH v2 1/2] x86/speculation: Allow enabling STIBP with legacy IBRS

[Pawan Gupta]

On Tue, Feb 21, 2023 at 07:49:07PM +0100, KP Singh wrote:
> Setting the IBRS bit implicitly enables STIBP to protect against
> cross-thread branch target injection. With enhanced IBRS, the bit it set
> once and is not cleared again. However, on CPUs with just legacy IBRS,
> IBRS bit set on user -> kernel and cleared on kernel -> user (a.k.a
> KERNEL_IBRS). Clearing this bit also disables the implicitly enabled
> STIBP, thus requiring some form of cross-thread protection in userspace.
> 
> Enable STIBP, either opt-in via prctl or seccomp, or always on depending
> on the choice of mitigation selected via spectre_v2_user.
> 
> Reported-by: José Oliveira <joseloliveira11@xxxxxxxxx>
> Reported-by: Rodrigo Branco <rodrigo@xxxxxxxxxxxxxxxxx>
> Reviewed-by: Alexandra Sandulescu <aesa@xxxxxxxxxx>
> Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
> ---
>  arch/x86/kernel/cpu/bugs.c | 33 ++++++++++++++++++++++-----------
>  1 file changed, 22 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 85168740f76a..5be6075d8e36 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1124,14 +1124,30 @@ spectre_v2_parse_user_cmdline(void)
>  	return SPECTRE_V2_USER_CMD_AUTO;
>  }
>  
> -static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
> +static inline bool spectre_v2_in_eibrs_mode(enum spectre_v2_mitigation mode)
>  {
> -	return mode == SPECTRE_V2_IBRS ||
> -	       mode == SPECTRE_V2_EIBRS ||
> +	return mode == SPECTRE_V2_EIBRS ||
>  	       mode == SPECTRE_V2_EIBRS_RETPOLINE ||
>  	       mode == SPECTRE_V2_EIBRS_LFENCE;
>  }
>  
> +static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
> +{
> +	return spectre_v2_in_eibrs_mode(mode) || mode == SPECTRE_V2_IBRS;
> +}
> +
> +static inline bool spectre_v2_user_needs_stibp(enum spectre_v2_mitigation mode)
> +{
> +	/*
> +	 * enhanced IBRS also protects against user-mode attacks as the IBRS bit

Maybe:
	 * Enhanced IBRS mode also protects against cross-thread user-to-user
	 * attacks as the IBRS bit

> +	 * remains always set which implicitly enables cross-thread protections.
> +	 * However, In legacy IBRS mode, the IBRS bit is set only in kernel
> +	 * and cleared on return to userspace. This disables the implicit
> +	 * cross-thread protections and STIBP is needed.
> +	 */
> +	return !spectre_v2_in_eibrs_mode(mode);
> +}
> +
>  static void __init
>  spectre_v2_user_select_mitigation(void)
>  {
> @@ -1193,13 +1209,8 @@ spectre_v2_user_select_mitigation(void)
>  			"always-on" : "conditional");
>  	}
>  
> -	/*
> -	 * If no STIBP, IBRS or enhanced IBRS is enabled, or SMT impossible,
> -	 * STIBP is not required.
> -	 */
> -	if (!boot_cpu_has(X86_FEATURE_STIBP) ||
> -	    !smt_possible ||
> -	    spectre_v2_in_ibrs_mode(spectre_v2_enabled))
> +	if (!boot_cpu_has(X86_FEATURE_STIBP) || !smt_possible ||
> +	    !spectre_v2_user_needs_stibp(spectre_v2_enabled))

As pointed out in other discussions, it will be great if can get rid of
eIBRS check, and do what the user asked for; or atleast print a warning
about not setting STIBP bit explicitly.

>  		return;
>  
>  	/*
> @@ -2327,7 +2338,7 @@ static ssize_t mmio_stale_data_show_state(char *buf)
>  
>  static char *stibp_state(void)
>  {
> -	if (spectre_v2_in_ibrs_mode(spectre_v2_enabled))
> +	if (!spectre_v2_user_needs_stibp(spectre_v2_enabled))

Decoupling STIBP and eIBRS will also get rid of this check.

>  		return "";