[PATCH v2 5/5] 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.

Rather than reinventing all of the cpu feature query code, this fixes
native_cpuid to work in PIC objects.

I haven't combined it with boot/cpuflags.c's cpuid implementation:
including asm/processor.h from boot/cpuflags.c results in a flood of
unrelated errors, and fixing it might be messy.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---
 arch/x86/boot/compressed/aslr.c  | 27 +++++++++++++++++++++++++++
 arch/x86/include/asm/processor.h | 21 ++++++++++++++++++---
 2 files changed, 45 insertions(+), 3 deletions(-)

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);
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index a4ea023..6096f3c 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -189,10 +189,25 @@ static inline int have_cpuid_p(void)
 static inline void native_cpuid(unsigned int *eax, unsigned int *ebx,
 				unsigned int *ecx, unsigned int *edx)
 {
-	/* ecx is often an input as well as an output. */
-	asm volatile("cpuid"
+	/*
+	 * This function can be used from the boot code, so it needs
+	 * to avoid using EBX in constraints in PIC mode.
+	 *
+	 * ecx is often an input as well as an output.
+	 */
+	asm volatile(".ifnc %%ebx,%1 ; .ifnc %%rbx,%1           \n\t"
+		     "movl  %%ebx,%1                            \n\t"
+		     ".endif ; .endif                           \n\t"
+		     "cpuid					\n\t"
+		     ".ifnc %%ebx,%1 ; .ifnc %%rbx,%1           \n\t"
+		     "xchgl %%ebx,%1                            \n\t"
+		     ".endif ; .endif"
 	    : "=a" (*eax),
-	      "=b" (*ebx),
+#if defined(__i386__) && defined(__PIC__)
+	      "=r" (*ebx),	/* gcc won't let us use ebx */
+#else
+	      "=b" (*ebx),	/* ebx is okay */
+#endif
 	      "=c" (*ecx),
 	      "=d" (*edx)
 	    : "0" (*eax), "2" (*ecx)
-- 
1.9.3

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