[v2] KVM: x86: Fix single-step debugging
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Message ID 1548074920-10651-1-git-send-email-alex.popov@linux.com
State New
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  • [v2] KVM: x86: Fix single-step debugging
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Commit Message

Alexander Popov Jan. 21, 2019, 12:48 p.m. UTC
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@linux.com>
---
 arch/x86/kvm/x86.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

Comments

Greg KH Jan. 21, 2019, 2:24 p.m. UTC | #1
On Mon, Jan 21, 2019 at 03:48:40PM +0300, 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@linux.com>
> ---
>  arch/x86/kvm/x86.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>
Alexander Popov Jan. 24, 2019, 8:41 a.m. UTC | #2
On 21.01.2019 15:48, 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.

Hello everyone!

Just a friendly ping about this fix.
Looking forward to the feedback.

Best regards,
Alexander
Paolo Bonzini Jan. 25, 2019, 5:38 p.m. UTC | #3
On 21/01/19 15:24, Greg KH wrote:
> On Mon, Jan 21, 2019 at 03:48:40PM +0300, 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@linux.com>
>> ---
>>  arch/x86/kvm/x86.c | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> <formletter>
> 
> This is not the correct way to submit patches for inclusion in the
> stable kernel tree.  Please read:
>     https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
> for how to do this properly.
> 
> </formletter>
> 

Yup, will take care of doing this properly. :)

Paolo
Paolo Bonzini Jan. 25, 2019, 5:52 p.m. UTC | #4
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@linux.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
Jim Mattson Feb. 5, 2019, 10:55 p.m. UTC | #5
On Fri, Jan 25, 2019 at 9:52 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> 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@linux.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

This patch seems to imply that userspace is expected to set the
guest's RFLAGS.TF when requesting KVM_GUEST_SINGLESTEP, but the
documentation for KVM_GUEST_SINGLESTEP is not clear about this. Can
someone clarify how KVM_GUEST_SINGLESTEP is supposed to work?

Patch
diff mbox series

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)