linux-kernel - [PATCH v3 5/6] x86/pti: Do not enable PTI on processors which are not vulnerable to Meltdown

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Date:   Wed, 24 Jan 2018 16:57:04 +0000
From:   David Woodhouse <dwmw@...zon.co.uk>
To:     arjan@...ux.intel.com, tglx@...utronix.de, karahmed@...zon.de,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com, bp@...en8.de, peterz@...radead.org,
        pbonzini@...hat.com, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org,
        dave.hansen@...el.com, gnomes@...rguk.ukuu.org.uk
Subject: [PATCH v3 5/6] x86/pti: Do not enable PTI on processors which are not vulnerable to Meltdown

Some old Atoms, anything in family 5 or 4, and newer CPUs when they advertise
the IA32_ARCH_CAPABILITIES MSR and it has the RDCL_NO bit set, are not vulnerable.

Roll the AMD exemption into the x86_match_cpu() table too.

Based on suggestions from Dave Hansen and Alan Cox.

Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
---
 arch/x86/kernel/cpu/common.c | 34 ++++++++++++++++++++++++++++++++--
 1 file changed, 32 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index e5d66e9..3bc8a1f 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -47,6 +47,8 @@
 #include <asm/pat.h>
 #include <asm/microcode.h>
 #include <asm/microcode_intel.h>
+#include <asm/intel-family.h>
+#include <asm/cpu_device_id.h>
 
 #ifdef CONFIG_X86_LOCAL_APIC
 #include <asm/uv/uv.h>
@@ -853,6 +855,35 @@ static void identify_cpu_without_cpuid(struct cpuinfo_x86 *c)
 #endif
 }
 
+static const __initdata struct x86_cpu_id cpu_no_meltdown[] = {
+	{ X86_VENDOR_AMD },
+	{ X86_VENDOR_INTEL, 6, INTEL_FAM6_ATOM_CEDARVIEW, X86_FEATURE_ANY },
+	{ X86_VENDOR_INTEL, 6, INTEL_FAM6_ATOM_CLOVERVIEW, X86_FEATURE_ANY },
+	{ X86_VENDOR_INTEL, 6, INTEL_FAM6_ATOM_LINCROFT, X86_FEATURE_ANY },
+	{ X86_VENDOR_INTEL, 6, INTEL_FAM6_ATOM_PENWELL, X86_FEATURE_ANY },
+	{ X86_VENDOR_INTEL, 6, INTEL_FAM6_ATOM_PINEVIEW, X86_FEATURE_ANY },
+	{ X86_VENDOR_ANY, 5 },
+	{ X86_VENDOR_ANY, 4 },
+	{}
+};
+
+static bool __init early_cpu_vulnerable_meltdown(struct cpuinfo_x86 *c)
+{
+	u64 ia32_cap = 0;
+
+	if (x86_match_cpu(cpu_no_meltdown))
+                return false;
+
+	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 false;
+
+	return true;
+}
+
 /*
  * Do minimum CPU detection early.
  * Fields really needed: vendor, cpuid_level, family, model, mask,
@@ -900,9 +931,8 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
 
 	setup_force_cpu_cap(X86_FEATURE_ALWAYS);
 
-	if (c->x86_vendor != X86_VENDOR_AMD)
+	if (early_cpu_vulnerable_meltdown(c))
 		setup_force_cpu_bug(X86_BUG_CPU_MELTDOWN);
-
 	setup_force_cpu_bug(X86_BUG_SPECTRE_V1);
 	setup_force_cpu_bug(X86_BUG_SPECTRE_V2);
 
-- 
2.7.4