From 4758c1f8ad5385e53ea85739ad9f6fa6dfc0abda Mon Sep 17 00:00:00 2001 From: Nagappan Ramasamy Palaniappan Date: Wed, 2 Aug 2023 12:55:46 +0000 Subject: [PATCH 3/5] x86/speculation: Allow enabling STIBP with legacy IBRS MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit When plain IBRS is enabled (not enhanced IBRS), the logic in spectre_v2_user_select_mitigation() determines that STIBP is not needed. The IBRS bit implicitly protects against cross-thread branch target injection. However, with legacy IBRS, the IBRS bit is cleared on returning to userspace for performance reasons which leaves userspace threads vulnerable to cross-thread branch target injection against which STIBP protects. Exclude IBRS from the spectre_v2_in_ibrs_mode() check to allow for enabling STIBP (through seccomp/prctl() by default or always-on, if selected by spectre_v2_user kernel cmdline parameter). [ bp: Massage. ] Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS") Reported-by: José Oliveira Reported-by: Rodrigo Branco Signed-off-by: KP Singh Signed-off-by: Borislav Petkov (AMD) Signed-off-by: Nagappan Ramasamy Palaniappan Reviewed-by: Laurence Rochfort --- arch/x86/kernel/cpu/bugs.c | 25 ++++++++++++++++++------- 1 file changed, 18 insertions(+), 7 deletions(-) diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index da7c361f4..e13c4ecdc 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -1095,14 +1095,18 @@ 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 void __init spectre_v2_user_select_mitigation(void) { @@ -1165,12 +1169,19 @@ spectre_v2_user_select_mitigation(void) } /* - * If no STIBP, IBRS or enhanced IBRS is enabled, or SMT impossible, - * STIBP is not required. + * If no STIBP, enhanced IBRS is enabled, or SMT impossible, STIBP + * is not required. + * + * Enhanced IBRS also protects against cross-thread branch target + * injection in user-mode as the IBRS bit remains always set which + * implicitly enables cross-thread protections. However, in legacy IBRS + * mode, the IBRS bit is set only on kernel entry and cleared on return + * to userspace. This disables the implicit cross-thread protection, + * so allow for STIBP to be selected in that case. */ 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; /* @@ -2295,7 +2306,7 @@ 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_in_eibrs_mode(spectre_v2_enabled)) return ""; switch (spectre_v2_user_stibp) { -- 2.31.1