Re: [PATCH V2 2/3] KVM: X86: Set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT

[Paolo Bonzini <pbonzini@redhat.com>]

On 09/08/21 19:43, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
> 
> Commit c77fb5fe6f03 ("KVM: x86: Allow the guest to run with dirty debug
> registers") allows the guest accessing to DRs without exiting when
> KVM_DEBUGREG_WONT_EXIT and we need to ensure that they are synchronized
> on entry to the guest---including DR6 that was not synced before the commit.
> 
> But the commit sets the hardware DR6 not only when KVM_DEBUGREG_WONT_EXIT,
> but also when KVM_DEBUGREG_BP_ENABLED.  The second case is unnecessary
> and just leads to a more case which leaks stale DR6 to the host which has
> to be resolved by unconditionally reseting DR6 in kvm_arch_vcpu_put().
> 
> We'd better to set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT,
> so that we can fine-grain control the cases when we need to reset it
> which is done in later patch.
> 
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---
>   arch/x86/kvm/x86.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ad47a09ce307..d2aa49722064 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9598,7 +9598,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>   		set_debugreg(vcpu->arch.eff_db[1], 1);
>   		set_debugreg(vcpu->arch.eff_db[2], 2);
>   		set_debugreg(vcpu->arch.eff_db[3], 3);
> -		set_debugreg(vcpu->arch.dr6, 6);
> +		/* When KVM_DEBUGREG_WONT_EXIT, dr6 is accessible in guest. */
> +		if (vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)
> +			set_debugreg(vcpu->arch.dr6, 6);
>   	} else if (unlikely(hw_breakpoint_active())) {
>   		set_debugreg(0, 7);
>   	}
> 

Even better, this should be moved to vmx.c's vcpu_enter_guest.  This
matches the handling in svm.c:

         /*
          * Run with all-zero DR6 unless needed, so that we can get the exact cause
          * of a #DB.
          */
         if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT))
                 svm_set_dr6(svm, vcpu->arch.dr6);
         else
                 svm_set_dr6(svm, DR6_ACTIVE_LOW);

That is,

     KVM: X86: Set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT
     
     Commit c77fb5fe6f03 ("KVM: x86: Allow the guest to run with dirty debug
     registers") allows the guest accessing to DRs without exiting when
     KVM_DEBUGREG_WONT_EXIT and we need to ensure that they are synchronized
     on entry to the guest---including DR6 that was not synced before the commit.
     
     But the commit sets the hardware DR6 not only when KVM_DEBUGREG_WONT_EXIT,
     but also when KVM_DEBUGREG_BP_ENABLED.  The second case is unnecessary
     and just leads to a more case which leaks stale DR6 to the host which has
     to be resolved by unconditionally reseting DR6 in kvm_arch_vcpu_put().
     
     Even if KVM_DEBUGREG_WONT_EXIT, however, setting the host DR6 only matters
     on VMX because SVM always uses the DR6 value from the VMCB.  So move this
     line to vmx.c and make it conditional on KVM_DEBUGREG_WONT_EXIT.
     
     Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index ae8e62df16dd..21a3ef3012cf 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6625,6 +6625,10 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
  		vmx->loaded_vmcs->host_state.cr4 = cr4;
  	}
  
+	/* When KVM_DEBUGREG_WONT_EXIT, dr6 is accessible in guest. */
+	if (vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)
+		set_debugreg(vcpu->arch.dr6, 6);
+
  	/* When single-stepping over STI and MOV SS, we must clear the
  	 * corresponding interruptibility bits in the guest state. Otherwise
  	 * vmentry fails as it then expects bit 14 (BS) in pending debug
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a111899ab2b4..fbc536b21585 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9597,7 +9597,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
  		set_debugreg(vcpu->arch.eff_db[1], 1);
  		set_debugreg(vcpu->arch.eff_db[2], 2);
  		set_debugreg(vcpu->arch.eff_db[3], 3);
-		set_debugreg(vcpu->arch.dr6, 6);
  	} else if (unlikely(hw_breakpoint_active())) {
  		set_debugreg(0, 7);
  	}

Paolo