Re: [PATCH] x86/bugs: Allow STIBP with IBRS

[KP Singh]

On Mon, Feb 20, 2023 at 10:27 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
> IBRS is only enabled in kernel space.  Since it's not enabled in user
> space, user space isn't protected from indirect branch prediction
> attacks from a sibling CPU thread.
>
> Allow STIBP to be enabled to protect against such attacks.
>
> Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")
> Reported-by: José Oliveira <joseloliveira11@xxxxxxxxx>
> Reported-by: Rodrigo Branco <rodrigo@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> ---
>  arch/x86/kernel/cpu/bugs.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 85168740f76a..b97c0d28e573 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1124,14 +1124,19 @@ 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;
>  }

There are no comments here, this code is in dire need for some
comments and explanation, I was trying something like:

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index bca0bd8f4846..3e04f9fa68a8 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1124,14 +1124,31 @@ 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;
 }

+/*
+ * In IBRS mode, the spectre_v2 mitigation is enabled only in kernel space with
+ * the IBRS bit being cleared on return to userspace due to performance
+ * overhead.
+ */
+static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
+{
+       return spectre_v2_in_eibrs_mode(mode) || mode == SPECTRE_V2_IBRS;
+}
+
+/*
+ * User mode protections are only available in eIBRS mode.
+ */
+static inline bool spectre_v2_user_needs_stibp(enum spectre_v2_mitigation mode)
+{
+       return !spectre_v2_in_eibrs_mode(mode);
+}
+
 static void __init
 spectre_v2_user_select_mitigation(void)
 {
@@ -1193,13 +1210,8 @@ 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))
                return;

        /*
@@ -2327,7 +2339,7 @@ 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))
                return "";

        switch (spectre_v2_user_stibp) {

Also Josh, is it okay for us to have a discussion and have me write
the patch as a v2? Your current patch does not even credit me at all.
Seems a bit unfair, but I don't really care. I was going to rev up the
patch with your suggestions.

>
> +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 void __init
>  spectre_v2_user_select_mitigation(void)
>  {
> @@ -1194,12 +1199,12 @@ spectre_v2_user_select_mitigation(void)
>         }
>
>         /*
> -        * If no STIBP, IBRS or enhanced IBRS is enabled, or SMT impossible,
> +        * If no STIBP, 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))
> +           spectre_v2_in_eibrs_mode(spectre_v2_enabled))
>                 return;
>
>         /*
> @@ -2327,9 +2332,6 @@ static ssize_t mmio_stale_data_show_state(char *buf)
>
>  static char *stibp_state(void)
>  {
> -       if (spectre_v2_in_ibrs_mode(spectre_v2_enabled))
> -               return "";
> -
>         switch (spectre_v2_user_stibp) {
>         case SPECTRE_V2_USER_NONE:
>                 return ", STIBP: disabled";
> --
> 2.39.1
>