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

[Jan Kiszka]

On 16.03.21 11:59, Maxim Levitsky wrote:
> 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.

I've triggered it recently while debugging a guest, that's why I got
aware of the code path. Long ago, all this used to work (soft BPs,
single-stepping etc.)

> 
> 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.

You have no issue with interactive debugging when NOT using gdb scripts
/ lx-symbol?

Jan

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