lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:   Tue, 10 Aug 2021 01:43:06 +0800
From:   Lai Jiangshan <>
Cc:     Lai Jiangshan <>,
        Paolo Bonzini <>,
        Sean Christopherson <>,
        Vitaly Kuznetsov <>,
        Wanpeng Li <>,
        Jim Mattson <>,
        Joerg Roedel <>,
        Thomas Gleixner <>,
        Ingo Molnar <>, Borislav Petkov <>,, "H. Peter Anvin" <>,
Subject: [PATCH V2 2/3] KVM: X86: Set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT

From: Lai Jiangshan <>

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 <>
 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);

Powered by blists - more mailing lists