x86/speculation: Disable RRSBA behavior

Some Intel processors may use alternate predictors for RETs on
RSB-underflow. This condition may be vulnerable to Branch History
Injection (BHI) and intramode-BTI.

Kernel earlier added spectre_v2 mitigation modes (eIBRS+Retpolines,
eIBRS+LFENCE, Retpolines) which protect indirect CALLs and JMPs against
such attacks. However, on RSB-underflow, RET target prediction may
fallback to alternate predictors. As a result, RET's predicted target
may get influenced by branch history.

A new MSR_IA32_SPEC_CTRL bit (RRSBA_DIS_S) controls this fallback
behavior when in kernel mode. When set, RETs will not take predictions
from alternate predictors, hence mitigating RETs as well. Support for
this is enumerated by CPUID.7.2.EDX[RRSBA_CTRL] (bit2).

For spectre v2 mitigation, when a user selects a mitigation that
protects indirect CALLs and JMPs against BHI and intramode-BTI, set
RRSBA_DIS_S also to protect RETs for RSB-underflow case.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>

1 files changed, 26 insertions, 0 deletions

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index d26c57d98b98..0dd04713434b 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1318,6 +1318,22 @@ static enum spectre_v2_mitigation __init spectre_v2_select_retpoline(void)
 	return SPECTRE_V2_RETPOLINE;
 }
 
+/* Disable in-kernel use of non-RSB RET predictors */
+static void __init spec_ctrl_disable_kernel_rrsba(void)
+{
+	u64 ia32_cap;
+
+	if (!boot_cpu_has(X86_FEATURE_RRSBA_CTRL))
+		return;
+
+	ia32_cap = x86_read_arch_cap_msr();
+
+	if (ia32_cap & ARCH_CAP_RRSBA) {
+		x86_spec_ctrl_base |= SPEC_CTRL_RRSBA_DIS_S;
+		write_spec_ctrl_current(x86_spec_ctrl_base, true);
+	}
+}
+
 static void __init spectre_v2_select_mitigation(void)
 {
 	enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
@@ -1412,6 +1428,16 @@ static void __init spectre_v2_select_mitigation(void)
 		break;
 	}
 
+	/*
+	 * Disable alternate RSB predictions in kernel when indirect CALLs and
+	 * JMPs gets protection against BHI and Intramode-BTI, but RET
+	 * prediction from a non-RSB predictor is still a risk.
+	 */
+	if (mode == SPECTRE_V2_EIBRS_LFENCE ||
+	    mode == SPECTRE_V2_EIBRS_RETPOLINE ||
+	    mode == SPECTRE_V2_RETPOLINE)
+		spec_ctrl_disable_kernel_rrsba();
+
 	spectre_v2_enabled = mode;
 	pr_info("%s\n", spectre_v2_strings[mode]);