Re: [PATCH 2/3] KVM: x86: guest debug: don't inject interrupts while single stepping

[Maxim Levitsky]

On Tue, 2021-03-16 at 10:16 +0100, Jan Kiszka wrote:
> On 16.03.21 00:37, Sean Christopherson wrote:
> > On Tue, Mar 16, 2021, Maxim Levitsky wrote:
> > > This change greatly helps with two issues:
> > > 
> > > * Resuming from a breakpoint is much more reliable.
> > > 
> > >   When resuming execution from a breakpoint, with interrupts enabled, more often
> > >   than not, KVM would inject an interrupt and make the CPU jump immediately to
> > >   the interrupt handler and eventually return to the breakpoint, to trigger it
> > >   again.
> > > 
> > >   From the user point of view it looks like the CPU never executed a
> > >   single instruction and in some cases that can even prevent forward progress,
> > >   for example, when the breakpoint is placed by an automated script
> > >   (e.g lx-symbols), which does something in response to the breakpoint and then
> > >   continues the guest automatically.
> > >   If the script execution takes enough time for another interrupt to arrive,
> > >   the guest will be stuck on the same breakpoint RIP forever.
> > > 
> > > * Normal single stepping is much more predictable, since it won't land the
> > >   debugger into an interrupt handler, so it is much more usable.
> > > 
> > >   (If entry to an interrupt handler is desired, the user can still place a
> > >   breakpoint at it and resume the guest, which won't activate this workaround
> > >   and let the gdb still stop at the interrupt handler)
> > > 
> > > Since this change is only active when guest is debugged, it won't affect
> > > KVM running normal 'production' VMs.
> > > 
> > > 
> > > Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> > > Tested-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> > > ---
> > >  arch/x86/kvm/x86.c | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > > 
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index a9d95f90a0487..b75d990fcf12b 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -8458,6 +8458,12 @@ static void inject_pending_event(struct kvm_vcpu *vcpu, bool *req_immediate_exit
> > >  		can_inject = false;
> > >  	}
> > >  
> > > +	/*
> > > +	 * Don't inject interrupts while single stepping to make guest debug easier
> > > +	 */
> > > +	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
> > > +		return;
> > 
> > Is this something userspace can deal with?  E.g. disable IRQs and/or set NMI
> > blocking at the start of single-stepping, unwind at the end?  Deviating this far
> > from architectural behavior will end in tears at some point.
> > 
> 
> Does this happen to address this suspicious workaround in the kernel?
> 
>         /*
>          * The kernel doesn't use TF single-step outside of:
>          *
>          *  - Kprobes, consumed through kprobe_debug_handler()
>          *  - KGDB, consumed through notify_debug()
>          *
>          * So if we get here with DR_STEP set, something is wonky.
>          *
>          * A known way to trigger this is through QEMU's GDB stub,
>          * which leaks #DB into the guest and causes IST recursion.
>          */
>         if (WARN_ON_ONCE(dr6 & DR_STEP))
>                 regs->flags &= ~X86_EFLAGS_TF;
> 
> (arch/x86/kernel/traps.c, exc_debug_kernel)
> 
> I wonder why this got merged while no one fixed QEMU/KVM, for years? Oh,
> yeah, question to myself as well, dancing around broken guest debugging
> for a long time while trying to fix other issues...

To be honest I didn't see that warning even once, but I can imagine KVM
leaking #DB due to bugs in that code. That area historically didn't receive
much attention since it can only be triggered by
KVM_GET/SET_GUEST_DEBUG which isn't used in production.

The only issue that I on the other hand  did
see which is mostly gdb fault is that it fails to remove a software breakpoint
when resuming over it, if that breakpoint's python handler messes up 
with gdb's symbols, which is what lx-symbols does.

And that despite the fact that lx-symbol doesn't mess with the object
(that is the kernel) where the breakpoint is defined.

Just adding/removing one symbol file is enough to trigger this issue.

Since lx-symbols already works this around when it reloads all symbols,
I extended that workaround to happen also when loading/unloading 
only a single symbol file.

Best regards,
	Maxim Levitsky

> 
> Jan
> 
> > > +
> > >  	/*
> > >  	 * Finally, inject interrupt events.  If an event cannot be injected
> > >  	 * due to architectural conditions (e.g. IF=0) a window-open exit
> > > -- 
> > > 2.26.2
> > >