[PATCH] x86/bugs: Allow STIBP with IBRS

[Josh Poimboeuf]

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;
 }
 
+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