Re: [PATCH] KVM: x86: Extend Spectre-v1 mitigation

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Fri, Nov 22, 2019 at 10:40:39AM -0800, Marios Pomonis wrote:
> From: Nick Finco <nifi@google.com>
> 
> This extends the Spectre-v1 mitigation introduced in
> commit 75f139aaf896 ("KVM: x86: Add memory barrier on vmcs field lookup")
> and commit 085331dfc6bb ("x86/kvm: Update spectre-v1 mitigation") in light
> of the Spectre-v1/L1TF combination described here:
> https://xenbits.xen.org/xsa/advisory-289.html
> 
> As reported in the link, an attacker can use the cache-load part of a
> Spectre-v1 gadget to bring memory into the L1 cache, then use L1TF to
> leak the loaded memory. Note that this attack is not fully mitigated by
> core scheduling; an attacker could employ L1TF on the same thread that
> loaded the memory in L1 instead of relying on neighboring hyperthreads.
> 
> This patch uses array_index_nospec() to prevent index computations from
> causing speculative loads into the L1 cache. These cases involve a
> bounds check followed by a memory read using the index; this is more
> common than the full Spectre-v1 pattern. In some cases, the index
> computation can be eliminated entirely by small amounts of refactoring.
> 
> Signed-off-by: Nick Finco <nifi@google.com>
> Signed-off-by: Marios Pomonis <pomonis@google.com>

+cc stable?

> Acked-by: Andrew Honig <ahonig@google.com>
> ---
>  arch/x86/kvm/emulate.c       | 11 ++++++++---
>  arch/x86/kvm/hyperv.c        | 10 ++++++----
>  arch/x86/kvm/i8259.c         |  6 +++++-
>  arch/x86/kvm/ioapic.c        | 15 +++++++++------
>  arch/x86/kvm/lapic.c         | 13 +++++++++----
>  arch/x86/kvm/mtrr.c          |  8 ++++++--
>  arch/x86/kvm/pmu.h           | 18 ++++++++++++++----
>  arch/x86/kvm/vmx/pmu_intel.c | 24 ++++++++++++++++--------
>  arch/x86/kvm/vmx/vmx.c       | 22 ++++++++++++++++------
>  arch/x86/kvm/x86.c           | 18 ++++++++++++++----
>  10 files changed, 103 insertions(+), 42 deletions(-)

Can you split this up into multiple patches?  I assume this needs to be
backported to stable, and throwing everything into a single patch will
make the process unnecessarily painful.  Reviewing things would also be
a lot easier.

...

> @@ -5828,6 +5836,8 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>  	u32 exit_reason = vmx->exit_reason;
> +	u32 bounded_exit_reason = array_index_nospec(exit_reason,
> +						kvm_vmx_max_exit_handlers);
>  	u32 vectoring_info = vmx->idt_vectoring_info;
>  
>  	trace_kvm_exit(exit_reason, vcpu, KVM_ISA_VMX);
> @@ -5911,7 +5921,7 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu)
>  	}
>  
>  	if (exit_reason < kvm_vmx_max_exit_handlers
> -	    && kvm_vmx_exit_handlers[exit_reason]) {
> +	    && kvm_vmx_exit_handlers[bounded_exit_reason]) {
>  #ifdef CONFIG_RETPOLINE
>  		if (exit_reason == EXIT_REASON_MSR_WRITE)
>  			return kvm_emulate_wrmsr(vcpu);
> @@ -5926,7 +5936,7 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu)
>  		else if (exit_reason == EXIT_REASON_EPT_MISCONFIG)
>  			return handle_ept_misconfig(vcpu);
>  #endif
> -		return kvm_vmx_exit_handlers[exit_reason](vcpu);
> +		return kvm_vmx_exit_handlers[bounded_exit_reason](vcpu);
>  	} else {
>  		vcpu_unimpl(vcpu, "vmx: unexpected exit reason 0x%x\n",
>  				exit_reason);

Oof, using exit_reason for the comparison is subtle.  Rather than
precompute the bounded exit reason, what about refactoring the code so
that the array_index_nospec() tomfoolery can be done after the actual
bounds check?  This would also avoid the nospec stuff when using the
direct retpoline handling.

---
 arch/x86/kvm/vmx/vmx.c | 56 ++++++++++++++++++++++--------------------
 1 file changed, 30 insertions(+), 26 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index d39475e2d44e..14c2efd66300 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5910,34 +5910,38 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu)
 		}
 	}
 
-	if (exit_reason < kvm_vmx_max_exit_handlers
-	    && kvm_vmx_exit_handlers[exit_reason]) {
+	if (exit_reason >= kvm_vmx_max_exit_handlers)
+		goto unexpected_vmexit;
+
 #ifdef CONFIG_RETPOLINE
-		if (exit_reason == EXIT_REASON_MSR_WRITE)
-			return kvm_emulate_wrmsr(vcpu);
-		else if (exit_reason == EXIT_REASON_PREEMPTION_TIMER)
-			return handle_preemption_timer(vcpu);
-		else if (exit_reason == EXIT_REASON_PENDING_INTERRUPT)
-			return handle_interrupt_window(vcpu);
-		else if (exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT)
-			return handle_external_interrupt(vcpu);
-		else if (exit_reason == EXIT_REASON_HLT)
-			return kvm_emulate_halt(vcpu);
-		else if (exit_reason == EXIT_REASON_EPT_MISCONFIG)
-			return handle_ept_misconfig(vcpu);
+	if (exit_reason == EXIT_REASON_MSR_WRITE)
+		return kvm_emulate_wrmsr(vcpu);
+	else if (exit_reason == EXIT_REASON_PREEMPTION_TIMER)
+		return handle_preemption_timer(vcpu);
+	else if (exit_reason == EXIT_REASON_PENDING_INTERRUPT)
+		return handle_interrupt_window(vcpu);
+	else if (exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT)
+		return handle_external_interrupt(vcpu);
+	else if (exit_reason == EXIT_REASON_HLT)
+		return kvm_emulate_halt(vcpu);
+	else if (exit_reason == EXIT_REASON_EPT_MISCONFIG)
+		return handle_ept_misconfig(vcpu);
 #endif
-		return kvm_vmx_exit_handlers[exit_reason](vcpu);
-	} else {
-		vcpu_unimpl(vcpu, "vmx: unexpected exit reason 0x%x\n",
-				exit_reason);
-		dump_vmcs();
-		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
-		vcpu->run->internal.suberror =
-			KVM_INTERNAL_ERROR_UNEXPECTED_EXIT_REASON;
-		vcpu->run->internal.ndata = 1;
-		vcpu->run->internal.data[0] = exit_reason;
-		return 0;
-	}
+
+	exit_reason = array_index_nospec(exit_reason, kvm_vmx_max_exit_handlers);
+	if (!kvm_vmx_exit_handlers[exit_reason])
+		goto unexpected_vmexit;
+
+	return kvm_vmx_exit_handlers[exit_reason](vcpu);
+
+unexpected_vmexit:
+	vcpu_unimpl(vcpu, "vmx: unexpected exit reason 0x%x\n", exit_reason);
+	dump_vmcs();
+	vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
+	vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_UNEXPECTED_EXIT_REASON;
+	vcpu->run->internal.ndata = 1;
+	vcpu->run->internal.data[0] = exit_reason;
+	return 0;
 }
 
 /*
-- 
2.24.0