[PATCH 4/4] x86,kaslr: Use MSR_KVM_GET_RNG_SEED for KASLR if available

[Andy Lutomirski]

It's considerably better than any of the alternatives on KVM.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---
 arch/x86/boot/compressed/aslr.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/arch/x86/boot/compressed/aslr.c b/arch/x86/boot/compressed/aslr.c
index fc6091a..8583f0e 100644
--- a/arch/x86/boot/compressed/aslr.c
+++ b/arch/x86/boot/compressed/aslr.c
@@ -5,6 +5,8 @@
 #include <asm/archrandom.h>
 #include <asm/e820.h>
 
+#include <uapi/asm/kvm_para.h>
+
 #include <generated/compile.h>
 #include <linux/module.h>
 #include <linux/uts.h>
@@ -15,6 +17,22 @@
 static const char build_str[] = UTS_RELEASE " (" LINUX_COMPILE_BY "@"
 		LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION;
 
+static bool kvm_para_has_feature(unsigned int feature)
+{
+	u32 kvm_base;
+	u32 features;
+
+	if (!has_cpuflag(X86_FEATURE_HYPERVISOR))
+		return false;
+
+	kvm_base = hypervisor_cpuid_base("KVMKVMKVM\0\0\0", KVM_CPUID_FEATURES);
+	if (!kvm_base)
+		return false;
+
+	features = cpuid_eax(kvm_base | KVM_CPUID_FEATURES);
+	return features & (1UL << feature);
+}
+
 #define I8254_PORT_CONTROL	0x43
 #define I8254_PORT_COUNTER0	0x40
 #define I8254_CMD_READBACK	0xC0
@@ -81,6 +99,15 @@ static unsigned long get_random_long(void)
 		}
 	}
 
+	if (kvm_para_has_feature(KVM_FEATURE_GET_RNG_SEED)) {
+		u64 seed;
+
+		debug_putstr(" MSR_KVM_GET_RNG_SEED");
+		rdmsrl(MSR_KVM_GET_RNG_SEED, seed);
+		random ^= (unsigned long)seed;
+		use_i8254 = false;
+	}
+
 	if (has_cpuflag(X86_FEATURE_TSC)) {
 		debug_putstr(" RDTSC");
 		rdtscll(raw);
-- 
1.9.3

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