KVM: X86: Disable hardware breakpoints unconditionally before kvm_x86->run()

commit f85d40160691881a17a397c448d799dfc90987ba upstream.

When the host is using debug registers but the guest is not using them
nor is the guest in guest-debug state, the kvm code does not reset
the host debug registers before kvm_x86->run().  Rather, it relies on
the hardware vmentry instruction to automatically reset the dr7 registers
which ensures that the host breakpoints do not affect the guest.

This however violates the non-instrumentable nature around VM entry
and exit; for example, when a host breakpoint is set on vcpu->arch.cr2,

Another issue is consistency.  When the guest debug registers are active,
the host breakpoints are reset before kvm_x86->run(). But when the
guest debug registers are inactive, the host breakpoints are delayed to
be disabled.  The host tracing tools may see different results depending
on what the guest is doing.

To fix the problems, we clear %db7 unconditionally before kvm_x86->run()
if the host has set any breakpoints, no matter if the guest is using
them or not.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Message-Id: <20210628172632.81029-1-jiangshanlai@gmail.com>
Cc: stable@vger.kernel.org
[Only clear %db7 instead of reloading all debug registers. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4b8c83100fad55b24ebc987a4219fd8b50328f98..42000fffe77ea7a7b6aa9effc393c5a1a02eebec 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6724,6 +6724,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
                set_debugreg(vcpu->arch.eff_db[3], 3);
                set_debugreg(vcpu->arch.dr6, 6);
                vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
+       } else if (unlikely(hw_breakpoint_active())) {
+               set_debugreg(0, 7);
        }
 
        kvm_x86_ops->run(vcpu);