[MODERATED] [PATCH v5 07/11] [PATCH v5 07/10] Linux Patch #7

[konrad.wilk@oracle.com]

Intel CPUs expose methods to:

 - Detect whether Reduced Data Speculation capability is available via
   CPUID.7.0.EDX[31],

 - The SPEC_CTRL MSR(0x48), bit 2 set to enable Reduced Data Speculation.

 - MSR_IA32_ARCH_CAPABILITIES, Bit(4) no need to enable Reduced Data Speculation.

With that in mind if spec_store_bypass_disable=[auto,on] is selected we will
set at boot-time the SPEC_CTRL MSR to enable Reduced Data Speculation if the
platform requires it.

Note that this does not fix the KVM case where the SPEC_CTRL
is exposed to guests who can muck with, see patch titled :
 KVM/SVM/VMX/x86/spectre_v2: Support the combination of guest IBRS and ours.

And for the firmware (IBRS to be set), see patch titled:
 x86/spectre_v2: Read SPEC_CTRL MSR during boot and re-use reserved bits

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---

v1.2: Expand on the commit description
  s/md_v4/mdd/
  s/spec_ctrl_msr_on/spec_ctrl_priv/
  s/spec_ctrl_msr_off/spec_ctrp_unpriv/

v1.3:
 - Add comment about privilege level changes.

v1.4: Simplify and incorporate various suggestions from Jon Masters
 - Export a single x86_spec_ctrl_base value with initial bits

v2: Rip out the c_fix_cpu.
 Depend on synthetic CPU flag
v3: Move the generic_identify to be done _after_ we figure out whether
  we can do the mitigation.
v4: s/MDD/RDS/
   s/Memory Disambiguation Disable/Reduced Data Speculation/
   Tweak the various 'disable', enabled now that it is called RDS.
   Set the x86_spec_ctrl with SPEC_CTRL_RDS if RDS is detected
   Fixup x86_set_spec_ctrl to deal with two Bitfields.
v5: s/X86_FEATURE_DISABLE_SSB/X86_FEATURE_SPEC_STORE_BYPASS_DISABLE/
   Also check MSR_IA32_ARCH_CAPABILITIES for Bit(4)
   Add documentation on what those three flags mean
   Add docs on why we set x86_spec_ctrl only on Intel
   Add extra check in ssb_parse_cmdline for RDS be available
   In init_intel drop the check for RDS as the X86_FEATURE_SPEC_STORE_BYPASS_DISABLE
    is implicitly set only iff RDS has been set in ssb_parse_cmdline.
---
 arch/x86/include/asm/cpufeatures.h |  1 +
 arch/x86/include/asm/msr-index.h   |  2 ++
 arch/x86/kernel/cpu/bugs.c         | 43 ++++++++++++++++++++++++++++++--------
 arch/x86/kernel/cpu/common.c       |  9 ++++----
 arch/x86/kernel/cpu/intel.c        |  6 ++++++
 5 files changed, 48 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index fc5a8378652e..914391814ba7 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -334,6 +334,7 @@
 #define X86_FEATURE_SPEC_CTRL		(18*32+26) /* "" Speculation Control (IBRS + IBPB) */
 #define X86_FEATURE_INTEL_STIBP		(18*32+27) /* "" Single Thread Indirect Branch Predictors */
 #define X86_FEATURE_ARCH_CAPABILITIES	(18*32+29) /* IA32_ARCH_CAPABILITIES MSR (Intel) */
+#define X86_FEATURE_RDS			(18*32+31) /* Reduced Data Speculation */
 
 /*
  * BUG word(s)
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index c9084dedfcfa..e4a6fb5f0888 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -42,6 +42,7 @@
 #define MSR_IA32_SPEC_CTRL		0x00000048 /* Speculation Control */
 #define SPEC_CTRL_IBRS			(1 << 0)   /* Indirect Branch Restricted Speculation */
 #define SPEC_CTRL_STIBP			(1 << 1)   /* Single Thread Indirect Branch Predictors */
+#define SPEC_CTRL_RDS			(1 << 2)   /* Reduced Data Speculation */
 
 #define MSR_IA32_PRED_CMD		0x00000049 /* Prediction Command */
 #define PRED_CMD_IBPB			(1 << 0)   /* Indirect Branch Prediction Barrier */
@@ -68,6 +69,7 @@
 #define MSR_IA32_ARCH_CAPABILITIES	0x0000010a
 #define ARCH_CAP_RDCL_NO		(1 << 0)   /* Not susceptible to Meltdown */
 #define ARCH_CAP_IBRS_ALL		(1 << 1)   /* Enhanced IBRS support */
+#define ARCH_CAP_RDS_NO			(1 << 4)   /* Not susceptible to speculative store bypass */
 
 #define MSR_IA32_BBL_CR_CTL		0x00000119
 #define MSR_IA32_BBL_CR_CTL3		0x0000011e
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index dff706bba3aa..99cdd44c20bc 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -37,8 +37,6 @@ u64 __read_mostly x86_spec_ctrl_base;
 
 void __init check_bugs(void)
 {
-	identify_boot_cpu();
-
 	/*
 	 * Read the SPEC_CTRL MSR to account for reserved bits which may have
 	 * unknown values.
@@ -46,11 +44,6 @@ void __init check_bugs(void)
 	if (boot_cpu_has(X86_FEATURE_IBRS))
 		rdmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
 
-	if (!IS_ENABLED(CONFIG_SMP)) {
-		pr_info("CPU: ");
-		print_cpu_info(&boot_cpu_data);
-	}
-
 	/* Select the proper spectre mitigation before patching alternatives */
 	spectre_v2_select_mitigation();
 
@@ -60,6 +53,18 @@ void __init check_bugs(void)
 	 */
 	ssb_select_mitigation();
 
+	/*
+	 * Need to do this _after_ spec_store_bypass_select_mitigation to
+	 * figure if any mitigation should be latched.
+	 */
+	identify_boot_cpu();
+
+	if (!IS_ENABLED(CONFIG_SMP)) {
+		pr_info("CPU: ");
+		print_cpu_info(&boot_cpu_data);
+	}
+
+
 #ifdef CONFIG_X86_32
 	/*
 	 * Check whether we are able to run this kernel safely on SMP.
@@ -117,7 +122,7 @@ static enum spectre_v2_mitigation spectre_v2_enabled = SPECTRE_V2_NONE;
 
 void x86_set_spec_ctrl(u64 val)
 {
-	if (val & ~(SPEC_CTRL_IBRS))
+	if (val & ~(SPEC_CTRL_IBRS | SPEC_CTRL_RDS))
 		WARN_ONCE(1, "SPEC_CTRL MSR value 0x%16llx is unknown.\n", val);
 	else
 		wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base | val);
@@ -399,6 +404,13 @@ static enum ssb_mitigation_cmd __init ssb_parse_cmdline(void)
 	if (!boot_cpu_has(X86_BUG_SPEC_STORE_BYPASS))
 		return SPEC_STORE_BYPASS_CMD_NONE;
 
+	/*
+	 * On Intel it is exposed in CPUID as Reduced Data Speculation
+	 * bit.
+	 */
+	if (!boot_cpu_has(X86_FEATURE_RDS))
+		return SPEC_STORE_BYPASS_CMD_NONE;
+
 	if (cmdline_find_option_bool(boot_command_line, "nospec_store_bypass_disable"))
 		return SPEC_STORE_BYPASS_CMD_NONE;
 	else {
@@ -446,8 +458,21 @@ static void __init ssb_select_mitigation(void)
 	ssb_mode = mode;
 	pr_info("%s\n", ssb_strings[mode]);
 
-	if (mode != SPEC_STORE_BYPASS_NONE)
+	/*
+	 * We have three CPU feature flags that are in play here:
+	 *  - X86_BUG_SPEC_STORE_BYPASS - CPU is susceptible.
+	 *  - X86_FEATURE_RDS - CPU is able to turn off speculative store bypass
+	 *  - X86_FEATURE_SPEC_STORE_BYPASS_DISABLE - engage the mitigation
+	 */
+	if (mode != SPEC_STORE_BYPASS_NONE) {
 		setup_force_cpu_cap(X86_FEATURE_SPEC_STORE_BYPASS_DISABLE);
+		/*
+		 * Intel uses the SPEC CTRL MSR Bit(2) for this.
+		 */
+		if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL)
+			x86_spec_ctrl_base |= SPEC_CTRL_RDS;
+
+	}
 }
 
 #undef pr_fmt
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 7ae1f2e6caf7..5d55423cce74 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -939,7 +939,11 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
 {
 	u64 ia32_cap = 0;
 
-	if (!x86_match_cpu(cpu_no_spec_store_bypass))
+	if (cpu_has(c, X86_FEATURE_ARCH_CAPABILITIES))
+		rdmsrl(MSR_IA32_ARCH_CAPABILITIES, ia32_cap);
+
+	if (!x86_match_cpu(cpu_no_spec_store_bypass) &&
+	   !(ia32_cap & ARCH_CAP_RDS_NO))
 		setup_force_cpu_bug(X86_BUG_SPEC_STORE_BYPASS);
 
 	if (x86_match_cpu(cpu_no_speculation))
@@ -951,9 +955,6 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
 	if (x86_match_cpu(cpu_no_meltdown))
 		return;
 
-	if (cpu_has(c, X86_FEATURE_ARCH_CAPABILITIES))
-		rdmsrl(MSR_IA32_ARCH_CAPABILITIES, ia32_cap);
-
 	/* Rogue Data Cache Load? No! */
 	if (ia32_cap & ARCH_CAP_RDCL_NO)
 		return;
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index c3af167d0a70..74fdc0fec2e0 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -17,6 +17,7 @@
 #include <asm/cpu.h>
 #include <asm/intel-family.h>
 #include <asm/microcode_intel.h>
+#include <asm/nospec-branch.h>
 #include <asm/hwcap2.h>
 #include <asm/elf.h>
 
@@ -189,6 +190,7 @@ static void early_init_intel(struct cpuinfo_x86 *c)
 		setup_clear_cpu_cap(X86_FEATURE_STIBP);
 		setup_clear_cpu_cap(X86_FEATURE_SPEC_CTRL);
 		setup_clear_cpu_cap(X86_FEATURE_INTEL_STIBP);
+		setup_clear_cpu_cap(X86_FEATURE_RDS);
 	}
 
 	/*
@@ -682,8 +684,12 @@ static void init_intel(struct cpuinfo_x86 *c)
 	init_intel_energy_perf(c);
 
 	init_intel_misc_features(c);
+
+	if (cpu_has(c, X86_FEATURE_SPEC_STORE_BYPASS_DISABLE))
+		x86_set_spec_ctrl(SPEC_CTRL_RDS);
 }
 
+
 #ifdef CONFIG_X86_32
 static unsigned int intel_size_cache(struct cpuinfo_x86 *c, unsigned int size)
 {
-- 
2.14.3