[PATCH v2 1/2] x86/speculation: Allow enabling STIBP with legacy IBRS

[KP Singh <kpsingh@kernel.org>]

Setting the IBRS bit implicitly enables STIBP to protect against
cross-thread branch target injection. With enhanced IBRS, the bit it set
once and is not cleared again. However, on CPUs with just legacy IBRS,
IBRS bit set on user -> kernel and cleared on kernel -> user (a.k.a
KERNEL_IBRS). Clearing this bit also disables the implicitly enabled
STIBP, thus requiring some form of cross-thread protection in userspace.

Enable STIBP, either opt-in via prctl or seccomp, or always on depending
on the choice of mitigation selected via spectre_v2_user.

Reported-by: José Oliveira <joseloliveira11@gmail.com>
Reported-by: Rodrigo Branco <rodrigo@kernelhacking.com>
Reviewed-by: Alexandra Sandulescu <aesa@google.com>
Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")
Cc: stable@vger.kernel.org
Signed-off-by: KP Singh <kpsingh@kernel.org>
---
 arch/x86/kernel/cpu/bugs.c | 33 ++++++++++++++++++++++-----------
 1 file changed, 22 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 85168740f76a..5be6075d8e36 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1124,14 +1124,30 @@ 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 inline bool spectre_v2_user_needs_stibp(enum spectre_v2_mitigation mode)
+{
+	/*
+	 * enhanced IBRS also protects against user-mode attacks as the IBRS bit
+	 * remains always set which implicitly enables cross-thread protections.
+	 * However, In legacy IBRS mode, the IBRS bit is set only in kernel
+	 * and cleared on return to userspace. This disables the implicit
+	 * cross-thread protections and STIBP is needed.
+	 */
+	return !spectre_v2_in_eibrs_mode(mode);
+}
+
 static void __init
 spectre_v2_user_select_mitigation(void)
 {
@@ -1193,13 +1209,8 @@ 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 +2338,7 @@ 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) {
-- 
2.39.2.637.g21b0678d19-goog