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

KP Singh
2023-02-20T18:34:16Z

On Mon, Feb 20, 2023 at 10:27 AM Josh Poimboeuf <jpoimboe@kernel.org> 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@gmail.com>
> Reported-by: Rodrigo Branco <rodrigo@kernelhacking.com>
> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> ---
> 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
>