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

Jan Kiszka
2021-03-16T09:18:10Z

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@redhat.com>
>> Tested-by: Stefano Garzarella <sgarzare@redhat.com>
>> ---
>> 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...

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
>>

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