Re: [PATCH] KVM: x86: Don't update RIP or do single-step on faulting emulation

[Jim Mattson]

On Fri, Aug 23, 2019 at 1:55 PM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> Don't advance RIP or inject a single-step #DB if emulation signals a
> fault.  This logic applies to all state updates that are conditional on
> clean retirement of the emulation instruction, e.g. updating RFLAGS was
> previously handled by commit 38827dbd3fb85 ("KVM: x86: Do not update
> EFLAGS on faulting emulation").
>
> Not advancing RIP is likely a nop, i.e. ctxt->eip isn't updated with
> ctxt->_eip until emulation "retires" anyways.  Skipping #DB injection
> fixes a bug reported by Andy Lutomirski where a #UD on SYSCALL due to
> invalid state with RFLAGS.RF=1 would loop indefinitely due to emulation
> overwriting the #UD with #DB and thus restarting the bad SYSCALL over
> and over.
>
> Cc: Nadav Amit <nadav.amit@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Fixes: 663f4c61b803 ("KVM: x86: handle singlestep during emulation")
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
>
> Note, this has minor conflict with my recent series to cleanup the
> emulator return flows[*].  The end result should look something like:
>
>                 if (!ctxt->have_exception ||
>                     exception_type(ctxt->exception.vector) == EXCPT_TRAP) {
>                         kvm_rip_write(vcpu, ctxt->eip);
>                         if (r && ctxt->tf)
>                                 r = kvm_vcpu_do_singlestep(vcpu);
>                         __kvm_set_rflags(vcpu, ctxt->eflags);
>                 }
>
> [*] https://lkml.kernel.org/r/20190823010709.24879-1-sean.j.christopherson@xxxxxxxxx
>
>  arch/x86/kvm/x86.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b4cfd786d0b6..d2962671c3d3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6611,12 +6611,13 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
>                 unsigned long rflags = kvm_x86_ops->get_rflags(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)
> -                       kvm_vcpu_do_singlestep(vcpu, &r);
>                 if (!ctxt->have_exception ||
> -                   exception_type(ctxt->exception.vector) == EXCPT_TRAP)
> +                   exception_type(ctxt->exception.vector) == EXCPT_TRAP) {

NYC, but...

I don't think this check for "exception_type" is quite right.  A
general detect fault (which can be synthesized by check_dr_read) is
mischaracterized by exception_type() as a trap. Or maybe I'm missing
something? (I often am.)

> +                       kvm_rip_write(vcpu, ctxt->eip);
> +                       if (r == EMULATE_DONE && ctxt->tf)
> +                               kvm_vcpu_do_singlestep(vcpu, &r);
>                         __kvm_set_rflags(vcpu, ctxt->eflags);
> +               }
>
>                 /*
>                  * For STI, interrupts are shadowed; so KVM_REQ_EVENT will
> --
> 2.22.0
>