[PATCH 1/4] x86,kvm: Add MSR_KVM_GET_RNG_SEED and a matching feature bit

Andy Lutomirski
2014-07-16T02:48:07Z

This adds a simple interface to allow a guest to request 64 bits of
host nonblocking entropy. This is independent of virtio-rng for a
couple of reasons:

- It's intended to be usable during early boot, when a trivial
synchronous interface is needed.

- virtio-rng gives blocking entropy, and making guest boot wait for
the host's /dev/random will cause problems.

MSR_KVM_GET_RNG_SEED is intended to provide 64 bits of best-effort
cryptographically secure data for use as a seed. It provides no
guarantee that the result contains any actual entropy.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
Documentation/virtual/kvm/cpuid.txt | 3 +++
arch/x86/include/uapi/asm/kvm_para.h | 2 ++
arch/x86/kvm/cpuid.c | 3 ++-
arch/x86/kvm/x86.c | 4 ++++
4 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
index 3c65feb..0ab043b 100644
--- a/Documentation/virtual/kvm/cpuid.txt
+++ b/Documentation/virtual/kvm/cpuid.txt
@@ -54,6 +54,9 @@ KVM_FEATURE_PV_UNHALT || 7 || guest checks this feature bit
|| || before enabling paravirtualized
|| || spinlock support.
------------------------------------------------------------------------------
+KVM_FEATURE_GET_RNG_SEED || 8 || host provides rng seed data via
+ || || MSR_KVM_GET_RNG_SEED.
+------------------------------------------------------------------------------
KVM_FEATURE_CLOCKSOURCE_STABLE_BIT || 24 || host will warn if no guest-side
|| || per-cpu warps are expected in
|| || kvmclock.
diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
index 94dc8ca..e2eaf93 100644
--- a/arch/x86/include/uapi/asm/kvm_para.h
+++ b/arch/x86/include/uapi/asm/kvm_para.h
@@ -24,6 +24,7 @@
#define KVM_FEATURE_STEAL_TIME 5
#define KVM_FEATURE_PV_EOI 6
#define KVM_FEATURE_PV_UNHALT 7
+#define KVM_FEATURE_GET_RNG_SEED 8

/* The last 8 bits are used to indicate how to interpret the flags field
* in pvclock structure. If no bits are set, all flags are ignored.
@@ -40,6 +41,7 @@
#define MSR_KVM_ASYNC_PF_EN 0x4b564d02
#define MSR_KVM_STEAL_TIME 0x4b564d03
#define MSR_KVM_PV_EOI_EN 0x4b564d04
+#define MSR_KVM_GET_RNG_SEED 0x4b564d05

struct kvm_steal_time {
__u64 steal;
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 38a0afe..40d6763 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -479,7 +479,8 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
(1 << KVM_FEATURE_ASYNC_PF) |
(1 << KVM_FEATURE_PV_EOI) |
(1 << KVM_FEATURE_CLOCKSOURCE_STABLE_BIT) |
- (1 << KVM_FEATURE_PV_UNHALT);
+ (1 << KVM_FEATURE_PV_UNHALT) |
+ (1 << KVM_FEATURE_GET_RNG_SEED);

if (sched_info_on())
entry->eax |= (1 << KVM_FEATURE_STEAL_TIME);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f644933..4e81853 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -48,6 +48,7 @@
#include <linux/pci.h>
#include <linux/timekeeper_internal.h>
#include <linux/pvclock_gtod.h>
+#include <linux/random.h>
#include <trace/events/kvm.h>

#define CREATE_TRACE_POINTS
@@ -2480,6 +2481,9 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata)
case MSR_KVM_PV_EOI_EN:
data = vcpu->arch.pv_eoi.msr_val;
break;
+ case MSR_KVM_GET_RNG_SEED:
+ get_random_bytes(&data, sizeof(data));
+ break;
case MSR_IA32_P5_MC_ADDR:
case MSR_IA32_P5_MC_TYPE:
case MSR_IA32_MCG_CAP:
--
1.9.3