[PATCH 5.15 77/89] x86/speculation: Disable RRSBA behavior

Greg Kroah-Hartman
2022-07-22T09:26:53Z

From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>

commit 4ad3278df6fe2b0852b00d5757fc2ccd8e92c26e upstream.

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>
[cascardo: no X86_FEATURE_INTEL_PPIN]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/include/asm/cpufeatures.h | 2 +-
arch/x86/include/asm/msr-index.h | 9 +++++++++
arch/x86/kernel/cpu/bugs.c | 26 ++++++++++++++++++++++++++
arch/x86/kernel/cpu/scattered.c | 1 +
tools/arch/x86/include/asm/msr-index.h | 9 +++++++++
5 files changed, 46 insertions(+), 1 deletion(-)

--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -295,7 +295,7 @@
#define X86_FEATURE_SGX1 (11*32+ 8) /* "" Basic SGX */
#define X86_FEATURE_SGX2 (11*32+ 9) /* "" SGX Enclave Dynamic Memory Management (EDMM) */
#define X86_FEATURE_ENTRY_IBPB (11*32+10) /* "" Issue an IBPB on kernel entry */
-/* FREE! (11*32+11) */
+#define X86_FEATURE_RRSBA_CTRL (11*32+11) /* "" RET prediction control */
#define X86_FEATURE_RETPOLINE (11*32+12) /* "" Generic Retpoline mitigation for Spectre variant 2 */
#define X86_FEATURE_RETPOLINE_LFENCE (11*32+13) /* "" Use LFENCE for Spectre variant 2 */
#define X86_FEATURE_RETHUNK (11*32+14) /* "" Use REturn THUNK */
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -51,6 +51,8 @@
#define SPEC_CTRL_STIBP BIT(SPEC_CTRL_STIBP_SHIFT) /* STIBP mask */
#define SPEC_CTRL_SSBD_SHIFT 2 /* Speculative Store Bypass Disable bit */
#define SPEC_CTRL_SSBD BIT(SPEC_CTRL_SSBD_SHIFT) /* Speculative Store Bypass Disable */
+#define SPEC_CTRL_RRSBA_DIS_S_SHIFT 6 /* Disable RRSBA behavior */
+#define SPEC_CTRL_RRSBA_DIS_S BIT(SPEC_CTRL_RRSBA_DIS_S_SHIFT)

#define MSR_IA32_PRED_CMD 0x00000049 /* Prediction Command */
#define PRED_CMD_IBPB BIT(0) /* Indirect Branch Prediction Barrier */
@@ -139,6 +141,13 @@
* bit available to control VERW
* behavior.
*/
+#define ARCH_CAP_RRSBA BIT(19) /*
+ * Indicates RET may use predictors
+ * other than the RSB. With eIBRS
+ * enabled predictions in kernel mode
+ * are restricted to targets in
+ * kernel.
+ */

#define MSR_IA32_FLUSH_CMD 0x0000010b
#define L1D_FLUSH BIT(0) /*
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1311,6 +1311,22 @@ static enum spectre_v2_mitigation __init
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();
@@ -1405,6 +1421,16 @@ static void __init spectre_v2_select_mit
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]);

--- a/arch/x86/kernel/cpu/scattered.c
+++ b/arch/x86/kernel/cpu/scattered.c
@@ -26,6 +26,7 @@ struct cpuid_bit {
static const struct cpuid_bit cpuid_bits[] = {
{ X86_FEATURE_APERFMPERF, CPUID_ECX, 0, 0x00000006, 0 },
{ X86_FEATURE_EPB, CPUID_ECX, 3, 0x00000006, 0 },
+ { X86_FEATURE_RRSBA_CTRL, CPUID_EDX, 2, 0x00000007, 2 },
{ X86_FEATURE_CQM_LLC, CPUID_EDX, 1, 0x0000000f, 0 },
{ X86_FEATURE_CQM_OCCUP_LLC, CPUID_EDX, 0, 0x0000000f, 1 },
{ X86_FEATURE_CQM_MBM_TOTAL, CPUID_EDX, 1, 0x0000000f, 1 },
--- a/tools/arch/x86/include/asm/msr-index.h
+++ b/tools/arch/x86/include/asm/msr-index.h
@@ -51,6 +51,8 @@
#define SPEC_CTRL_STIBP BIT(SPEC_CTRL_STIBP_SHIFT) /* STIBP mask */
#define SPEC_CTRL_SSBD_SHIFT 2 /* Speculative Store Bypass Disable bit */
#define SPEC_CTRL_SSBD BIT(SPEC_CTRL_SSBD_SHIFT) /* Speculative Store Bypass Disable */
+#define SPEC_CTRL_RRSBA_DIS_S_SHIFT 6 /* Disable RRSBA behavior */
+#define SPEC_CTRL_RRSBA_DIS_S BIT(SPEC_CTRL_RRSBA_DIS_S_SHIFT)

#define MSR_IA32_PRED_CMD 0x00000049 /* Prediction Command */
#define PRED_CMD_IBPB BIT(0) /* Indirect Branch Prediction Barrier */
@@ -138,6 +140,13 @@
* bit available to control VERW
* behavior.
*/
+#define ARCH_CAP_RRSBA BIT(19) /*
+ * Indicates RET may use predictors
+ * other than the RSB. With eIBRS
+ * enabled predictions in kernel mode
+ * are restricted to targets in
+ * kernel.
+ */

#define MSR_IA32_FLUSH_CMD 0x0000010b
#define L1D_FLUSH BIT(0) /*