[PATCH] x86/bugs: Allow STIBP with IBRS

Josh Poimboeuf
2023-02-20T18:27:25Z

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;
}

+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