2019-01-21 12:22:51

by Alexander Popov

Subject: [PATCH] KVM: x86: Fix single-step debugging

The single-step debugging of KVM guests on x86 is broken: if we run
gdb 'stepi' command at the breakpoint when the guest interrupts are
enabled, RIP always jumps to native_apic_mem_write(). Then other
nasty effects follow.

Long investigation showed that the commit c8401dda2f0a00cd25c0 (Jun 7 2017)
introduced the kvm_run.debug corruption: kvm_vcpu_do_singlestep() can
be called without X86_EFLAGS_TF set.

Let's fix it. Please consider that for -stable.
---
arch/x86/kvm/x86.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f049ecf..9686068 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6407,8 +6407,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
toggle_interruptibility(vcpu, ctxt->interruptibility);
vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
kvm_rip_write(vcpu, ctxt->eip);
- if (r == EMULATE_DONE &&
- (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)))
+ if (r == EMULATE_DONE && ctxt->tf)
kvm_vcpu_do_singlestep(vcpu, &r);
if (!ctxt->have_exception ||
exception_type(ctxt->exception.vector) == EXCPT_TRAP)
--
2.7.4

2019-01-21 12:48:36

by Alexander Popov

Subject: Re: [PATCH] KVM: x86: Fix single-step debugging

Oops. I've forgotten about checkpatch.pl.
I've fixed the commit message and will resend the patch shortly.

--
Alexander