Branch History Injection (BHI) attacks can be mitigated using the
BHI_DIS_S indirect predictor control bit located in MSR_IA32_SPEC_CTRL
register. Set BHI_DIS in MSR_IA32_SPC_CTRL to prevent predicted
targets of indirect branches executed in CPL0, CPL1, or CPL2 from
being selected based on branch history from branches executed in CPL3.
Support for this feature is enumerated by CPUID.7.2.EDX[BHI_CTRL] (bit 4).
Users wanting BHI protection can specify spectre_v2=eibrs,bhi_dis to
enable hardware BHI protections. On platforms where BHI protections
are not available in the hardware revert to eibrs,retpoline
mitigations.
Signed-off-by: Daniel Sneddon <daniel.sneddon@xxxxxxxxxxxxxxx>
---
Documentation/admin-guide/hw-vuln/spectre.rst | 5 ++
.../admin-guide/kernel-parameters.txt | 2 +
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/msr-index.h | 8 +++
arch/x86/include/asm/nospec-branch.h | 1 +
arch/x86/kernel/cpu/bugs.c | 53 +++++++++++++++++--
arch/x86/kernel/cpu/scattered.c | 1 +
tools/arch/x86/include/asm/msr-index.h | 8 +++
8 files changed, 76 insertions(+), 3 deletions(-)
diff --git a/Documentation/admin-guide/hw-vuln/spectre.rst b/Documentation/admin-guide/hw-vuln/spectre.rst
index 9e9556826450..cde2f860a7b7 100644
--- a/Documentation/admin-guide/hw-vuln/spectre.rst
+++ b/Documentation/admin-guide/hw-vuln/spectre.rst
@@ -605,6 +605,11 @@ kernel command line.
eibrs enhanced IBRS
eibrs,retpoline enhanced IBRS + Retpolines
eibrs,lfence enhanced IBRS + LFENCE
+ eibrs,bhi_dis enhanced IBRS + Hardware BHI prevention
+
+ Specifying eibrs,bhi_dis on hardware that is missing that
+ feature will cause the software to fall back to
+ "eibrs,retpoline" mode.
Not specifying this option is equivalent to
spectre_v2=auto.
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f2d26cb7e853..e0cb07b3f388 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5592,6 +5592,8 @@
eibrs - enhanced IBRS
eibrs,retpoline - enhanced IBRS + Retpolines
eibrs,lfence - enhanced IBRS + LFENCE
+ eibrs,bhi_dis - enhanced IBRS + Hardware BHI
+ prevention
ibrs - use IBRS to protect kernel
Not specifying this option is equivalent to
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 00f5227c8459..eab1baa8036f 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -302,6 +302,7 @@
#define X86_FEATURE_RETPOLINE_LFENCE (11*32+13) /* "" Use LFENCE for Spectre variant 2 */
#define X86_FEATURE_RETHUNK (11*32+14) /* "" Use REturn THUNK */
#define X86_FEATURE_UNRET (11*32+15) /* "" AMD BTB untrain return */
+#define X86_FEATURE_BHI_CTRL (11*32+16) /* "" Branch History Injection control */
/* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
#define X86_FEATURE_AVX_VNNI (12*32+ 4) /* AVX VNNI instructions */
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index cc615be27a54..55bfa2764033 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -53,6 +53,8 @@
#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 SPEC_CTRL_BHI_DIS_S_SHIFT 10 /* Disable Branch History Injection behavior */
+#define SPEC_CTRL_BHI_DIS_S BIT(SPEC_CTRL_BHI_DIS_S_SHIFT)
#define MSR_IA32_PRED_CMD 0x00000049 /* Prediction Command */
#define PRED_CMD_IBPB BIT(0) /* Indirect Branch Prediction Barrier */
@@ -150,6 +152,12 @@
* are restricted to targets in
* kernel.
*/
+#define ARCH_CAP_BHI_NO BIT(20) /*
+ * Predicted targets of CPL0
+ * indirect branches are not
+ * based on branch history from
+ * CPL3.
+ */
#define MSR_IA32_FLUSH_CMD 0x0000010b
#define L1D_FLUSH BIT(0) /*
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index bb05ed4f46bd..57c6fbf15579 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -237,6 +237,7 @@ enum spectre_v2_mitigation {
SPECTRE_V2_EIBRS,
SPECTRE_V2_EIBRS_RETPOLINE,
SPECTRE_V2_EIBRS_LFENCE,
+ SPECTRE_V2_EIBRS_BHI,
SPECTRE_V2_IBRS,
};
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 0dd04713434b..16e448f141e4 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -933,6 +933,7 @@ static void __init retbleed_select_mitigation(void)
case SPECTRE_V2_EIBRS:
case SPECTRE_V2_EIBRS_RETPOLINE:
case SPECTRE_V2_EIBRS_LFENCE:
+ case SPECTRE_V2_EIBRS_BHI:
retbleed_mitigation = RETBLEED_MITIGATION_EIBRS;
break;
default:
@@ -1016,6 +1017,7 @@ enum spectre_v2_mitigation_cmd {
SPECTRE_V2_CMD_EIBRS,
SPECTRE_V2_CMD_EIBRS_RETPOLINE,
SPECTRE_V2_CMD_EIBRS_LFENCE,
+ SPECTRE_V2_CMD_EIBRS_BHI,
SPECTRE_V2_CMD_IBRS,
};
@@ -1096,7 +1098,8 @@ static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
return mode == SPECTRE_V2_IBRS ||
mode == SPECTRE_V2_EIBRS ||
mode == SPECTRE_V2_EIBRS_RETPOLINE ||
- mode == SPECTRE_V2_EIBRS_LFENCE;
+ mode == SPECTRE_V2_EIBRS_LFENCE ||
+ mode == SPECTRE_V2_EIBRS_BHI;
}
static void __init
@@ -1198,6 +1201,7 @@ static const char * const spectre_v2_strings[] = {
[SPECTRE_V2_EIBRS] = "Mitigation: Enhanced IBRS",
[SPECTRE_V2_EIBRS_LFENCE] = "Mitigation: Enhanced IBRS + LFENCE",
[SPECTRE_V2_EIBRS_RETPOLINE] = "Mitigation: Enhanced IBRS + Retpolines",
+ [SPECTRE_V2_EIBRS_BHI] = "Mitigation: Enhanced IBRS + BHI disabled",
[SPECTRE_V2_IBRS] = "Mitigation: IBRS",
};
@@ -1215,6 +1219,7 @@ static const struct {
{ "eibrs", SPECTRE_V2_CMD_EIBRS, false },
{ "eibrs,lfence", SPECTRE_V2_CMD_EIBRS_LFENCE, false },
{ "eibrs,retpoline", SPECTRE_V2_CMD_EIBRS_RETPOLINE, false },
+ { "eibrs,bhi_dis", SPECTRE_V2_CMD_EIBRS_BHI, false },
{ "auto", SPECTRE_V2_CMD_AUTO, false },
{ "ibrs", SPECTRE_V2_CMD_IBRS, false },
};
@@ -1251,6 +1256,13 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
return SPECTRE_V2_CMD_AUTO;
}
+ if (cmd == SPECTRE_V2_CMD_EIBRS_BHI &&
+ !boot_cpu_has(X86_FEATURE_BHI_CTRL)) {
+ pr_err("%s selected but BHI not available. Switching to EIBRS_RETPOLINE\n",
+ mitigation_options[i].option);
+ cmd = SPECTRE_V2_CMD_EIBRS_RETPOLINE;
+ }
+
if ((cmd == SPECTRE_V2_CMD_RETPOLINE ||
cmd == SPECTRE_V2_CMD_RETPOLINE_LFENCE ||
cmd == SPECTRE_V2_CMD_RETPOLINE_GENERIC ||
@@ -1264,7 +1276,8 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
if ((cmd == SPECTRE_V2_CMD_EIBRS ||
cmd == SPECTRE_V2_CMD_EIBRS_LFENCE ||
- cmd == SPECTRE_V2_CMD_EIBRS_RETPOLINE) &&
+ cmd == SPECTRE_V2_CMD_EIBRS_RETPOLINE ||
+ cmd == SPECTRE_V2_CMD_EIBRS_BHI) &&
!boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) {
pr_err("%s selected but CPU doesn't have eIBRS. Switching to AUTO select\n",
mitigation_options[i].option);
@@ -1334,6 +1347,31 @@ static void __init spec_ctrl_disable_kernel_rrsba(void)
}
}
+/*
+ * This prevents predicted targets of indirect branches executed in CPL0, CPL1,
+ * or CPL2 from being selected based on branch history from branches executed in
+ * CPL3.
+ */
+static void __init spec_ctrl_bhi_dis(void)
+{
+ u64 ia32_cap;
+
+ if (!boot_cpu_has(X86_FEATURE_BHI_CTRL))
+ return;
+
+ ia32_cap = x86_read_arch_cap_msr();
+
+ /*
+ * On certain hardware there is no reason to set this bit in the MSR
+ * since it's ignored and BHI protections are always enabled anyway.
+ */
+ if (ia32_cap & ARCH_CAP_BHI_NO)
+ return;
+
+ x86_spec_ctrl_base |= SPEC_CTRL_BHI_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();
@@ -1398,6 +1436,10 @@ static void __init spectre_v2_select_mitigation(void)
case SPECTRE_V2_CMD_EIBRS_RETPOLINE:
mode = SPECTRE_V2_EIBRS_RETPOLINE;
break;
+
+ case SPECTRE_V2_CMD_EIBRS_BHI:
+ mode = SPECTRE_V2_EIBRS_BHI;
+ break;
}
if (mode == SPECTRE_V2_EIBRS && unprivileged_ebpf_enabled())
@@ -1426,6 +1468,10 @@ static void __init spectre_v2_select_mitigation(void)
case SPECTRE_V2_EIBRS_RETPOLINE:
setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
break;
+
+ case SPECTRE_V2_EIBRS_BHI:
+ spec_ctrl_bhi_dis();
+ break;
}
/*
@@ -1435,7 +1481,8 @@ static void __init spectre_v2_select_mitigation(void)
*/
if (mode == SPECTRE_V2_EIBRS_LFENCE ||
mode == SPECTRE_V2_EIBRS_RETPOLINE ||
- mode == SPECTRE_V2_RETPOLINE)
+ mode == SPECTRE_V2_RETPOLINE ||
+ mode == SPECTRE_V2_EIBRS_BHI)
spec_ctrl_disable_kernel_rrsba();
spectre_v2_enabled = mode;
diff --git a/arch/x86/kernel/cpu/scattered.c b/arch/x86/kernel/cpu/scattered.c
index fd44b54c90d5..9a8f0c086859 100644
--- a/arch/x86/kernel/cpu/scattered.c
+++ b/arch/x86/kernel/cpu/scattered.c
@@ -28,6 +28,7 @@ static const struct cpuid_bit cpuid_bits[] = {
{ X86_FEATURE_EPB, CPUID_ECX, 3, 0x00000006, 0 },
{ X86_FEATURE_INTEL_PPIN, CPUID_EBX, 0, 0x00000007, 1 },
{ X86_FEATURE_RRSBA_CTRL, CPUID_EDX, 2, 0x00000007, 2 },
+ { X86_FEATURE_BHI_CTRL, CPUID_EDX, 4, 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 },
diff --git a/tools/arch/x86/include/asm/msr-index.h b/tools/arch/x86/include/asm/msr-index.h
index 2eab6a3a8a8c..63f227aa9bf6 100644
--- a/tools/arch/x86/include/asm/msr-index.h
+++ b/tools/arch/x86/include/asm/msr-index.h
@@ -53,6 +53,8 @@
#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 SPEC_CTRL_BHI_DIS_S_SHIFT 10 /* Disable Branch History Injection behavior */
+#define SPEC_CTRL_BHI_DIS_S BIT(SPEC_CTRL_BHI_DIS_S_SHIFT)
#define MSR_IA32_PRED_CMD 0x00000049 /* Prediction Command */
#define PRED_CMD_IBPB BIT(0) /* Indirect Branch Prediction Barrier */
@@ -149,6 +151,12 @@
* are restricted to targets in
* kernel.
*/
+#define ARCH_CAP_BHI_NO BIT(20) /*
+ * Predicted targets of CPL0
+ * indirect branches are not
+ * based on branch history from
+ * CPL3.
+ */
#define MSR_IA32_FLUSH_CMD 0x0000010b
#define L1D_FLUSH BIT(0) /*
--
2.25.1
