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

[Andy Lutomirski]

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

EFLAGS.TF=1