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

[Paolo Bonzini]

On 09/08/21 19:43, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxxxx>
> ---
>   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@xxxxxxxxxx>

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