Re: [PATCH v5 4/5] x86,random,kvm: Use KVM_GET_RNG_SEED in arch_get_rng_seed

[Paolo Bonzini]

Il 24/07/2014 06:57, Andy Lutomirski ha scritto:
> This is a straightforward implementation: for each bit of internal
> RNG state, request one bit from KVM_GET_RNG_SEED.  This is done even
> if RDSEED/RDRAND worked, since KVM_GET_RNG_SEED is likely to provide
> cryptographically secure output even if the CPU's RNG is weak or
> compromised.
> 
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> ---
>  arch/x86/Kconfig                 |  4 ++++
>  arch/x86/include/asm/kvm_guest.h |  9 +++++++++
>  arch/x86/kernel/archrandom.c     | 25 ++++++++++++++++++++++++-
>  arch/x86/kernel/kvm.c            | 10 ++++++++++
>  4 files changed, 47 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index a8f749e..adfa09c 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -593,6 +593,7 @@ config KVM_GUEST
>  	bool "KVM Guest support (including kvmclock)"
>  	depends on PARAVIRT
>  	select PARAVIRT_CLOCK
> +	select ARCH_RANDOM
>  	default y
>  	---help---
>  	  This option enables various optimizations for running under the KVM
> @@ -1507,6 +1508,9 @@ config ARCH_RANDOM
>  	  If supported, this is a high bandwidth, cryptographically
>  	  secure hardware random number generator.
>  
> +	  This also enables paravirt RNGs such as KVM's if the relevant
> +	  PV guest support is enabled.
> +
>  config X86_SMAP
>  	def_bool y
>  	prompt "Supervisor Mode Access Prevention" if EXPERT
> diff --git a/arch/x86/include/asm/kvm_guest.h b/arch/x86/include/asm/kvm_guest.h
> index a92b176..8c4dbd5 100644
> --- a/arch/x86/include/asm/kvm_guest.h
> +++ b/arch/x86/include/asm/kvm_guest.h
> @@ -3,4 +3,13 @@
>  
>  int kvm_setup_vsyscall_timeinfo(void);
>  
> +#if defined(CONFIG_KVM_GUEST) && defined(CONFIG_ARCH_RANDOM)
> +extern bool kvm_get_rng_seed(u64 *rv);
> +#else
> +static inline bool kvm_get_rng_seed(u64 *rv)
> +{
> +	return false;
> +}
> +#endif
> +
>  #endif /* _ASM_X86_KVM_GUEST_H */
> diff --git a/arch/x86/kernel/archrandom.c b/arch/x86/kernel/archrandom.c
> index 47d13b0..8c8d021 100644
> --- a/arch/x86/kernel/archrandom.c
> +++ b/arch/x86/kernel/archrandom.c
> @@ -15,6 +15,7 @@
>   */
>  
>  #include <asm/archrandom.h>
> +#include <asm/kvm_guest.h>
>  
>  void arch_get_rng_seed(void *ctx,
>  		       void (*seed)(void *ctx, u32 data),
> @@ -22,7 +23,7 @@ void arch_get_rng_seed(void *ctx,
>  		       const char *log_prefix)
>  {
>  	int i;
> -	int rdseed_bits = 0, rdrand_bits = 0;
> +	int rdseed_bits = 0, rdrand_bits = 0, kvm_bits = 0;
>  	char buf[128] = "";
>  	char *msgptr = buf;
>  
> @@ -42,10 +43,32 @@ void arch_get_rng_seed(void *ctx,
>  #endif
>  	}
>  
> +	/*
> +	 * Use KVM_GET_RNG_SEED regardless of whether the CPU RNG
> +	 * worked, since it incorporates entropy unavailable to the CPU,
> +	 * and we shouldn't trust the hardware RNG more than we need to.
> +	 * We request enough bits for the entire internal RNG state,
> +	 * because there's no good reason not to.
> +	 */
> +	for (i = 0; i < bits_per_source; i += 64) {
> +		u64 rv;
> +
> +		if (kvm_get_rng_seed(&rv)) {
> +			seed(ctx, (u32)rv);
> +			seed(ctx, (u32)(rv >> 32));
> +			kvm_bits += 8 * sizeof(rv);
> +		} else {
> +			break;	/* If it fails once, it will keep failing. */
> +		}
> +	}
> +
>  	if (rdseed_bits)
>  		msgptr += sprintf(msgptr, ", %d bits from RDSEED", rdseed_bits);
>  	if (rdrand_bits)
>  		msgptr += sprintf(msgptr, ", %d bits from RDRAND", rdrand_bits);
> +	if (kvm_bits)
> +		msgptr += sprintf(msgptr, ", %d bits from KVM_GET_RNG_BITS",
> +				  kvm_bits);
>  	if (buf[0])
>  		pr_info("%s with %s\n", log_prefix, buf + 2);
>  }
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 3dd8e2c..bd8783a 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -416,6 +416,16 @@ void kvm_disable_steal_time(void)
>  	wrmsr(MSR_KVM_STEAL_TIME, 0, 0);
>  }
>  
> +bool kvm_get_rng_seed(u64 *v)
> +{
> +	/*
> +	 * Allow migration from a hypervisor with the GET_RNG_SEED
> +	 * feature to a hypervisor without it.
> +	 */
> +	return (kvm_para_has_feature(KVM_FEATURE_GET_RNG_SEED) &&
> +		rdmsrl_safe(MSR_KVM_GET_RNG_SEED, v) == 0);
> +}
> +
>  #ifdef CONFIG_SMP
>  static void __init kvm_smp_prepare_boot_cpu(void)
>  {
> 

Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
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