linux-kernel - [PATCH] KVM: x86: Fix single-step debugging

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Message-Id: <1548073224-5363-1-git-send-email-alex.popov@linux.com>
Date:   Mon, 21 Jan 2019 15:20:24 +0300
From:   Alexander Popov <alex.popov@...ux.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Nadav Amit <nadav.amit@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        H Peter Anvin <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        linux-kernel@...r.kernel.org,
        Alexander Popov <alex.popov@...ux.com>
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