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Subject[PATCH v5 5/5] x86,kaslr: Use MSR_KVM_GET_RNG_SEED for KASLR if available
Date
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.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
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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