linux-kernel - Re: [PATCH v2] KVM: x86: Fix single-step debugging

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Message-ID: <2cf80ef9-cacc-c730-7a9e-301703ddc80d@redhat.com>
Date:   Fri, 25 Jan 2019 18:52:15 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Alexander Popov <alex.popov@...ux.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
Subject: Re: [PATCH v2] KVM: x86: Fix single-step debugging

On 21/01/19 13:48, Alexander Popov wrote:
> 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 on Jun 7, 2017 the
> commit c8401dda2f0a00cd25c0 ("KVM: x86: fix singlestepping over syscall")
> 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.
> 
> Signed-off-by: Alexander Popov <alex.popov@...ux.com>
> ---
>  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)
> 

Queued, with Cc to stable and the right "Fixes" trailer.

Paolo