Re: [PATCH 3/3] KVM: x86: always stop emulation on page fault

[Sean Christopherson]

Actually adding Peng Hao and Yi Wang...

On Tue, Aug 27, 2019 at 07:50:30AM -0700, Sean Christopherson wrote:
> +Cc Peng Hao and Yi Wang
> 
> On Tue, Aug 27, 2019 at 01:07:09PM +0000, Jan Dakinevich wrote:
> > inject_emulated_exception() returns true if and only if nested page
> > fault happens. However, page fault can come from guest page tables
> > walk, either nested or not nested. In both cases we should stop an
> > attempt to read under RIP and give guest to step over its own page
> > fault handler.
> > 
> > Fixes: 6ea6e84 ("KVM: x86: inject exceptions produced by x86_decode_insn")
> > Cc: Denis Lunev <den@xxxxxxxxxxxxx>
> > Cc: Roman Kagan <rkagan@xxxxxxxxxxxxx>
> > Cc: Denis Plotnikov <dplotnikov@xxxxxxxxxxxxx>
> > Signed-off-by: Jan Dakinevich <jan.dakinevich@xxxxxxxxxxxxx>
> > ---
> >  arch/x86/kvm/x86.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 93b0bd4..45caa69 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -6521,8 +6521,10 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
> >  			if (reexecute_instruction(vcpu, cr2, write_fault_to_spt,
> >  						emulation_type))
> >  				return EMULATE_DONE;
> > -			if (ctxt->have_exception && inject_emulated_exception(vcpu))
> > +			if (ctxt->have_exception) {
> > +				inject_emulated_exception(vcpu);
> >  				return EMULATE_DONE;
> > +			}
> 
> 
> Yikes, this patch and the previous have quite the sordid history.
> 
> 
> The non-void return from inject_emulated_exception() was added by commit
> 
>   ef54bcfeea6c ("KVM: x86: skip writeback on injection of nested exception")
> 
> for the purpose of skipping writeback.  At the time, the above blob in the
> decode flow didn't exist.
> 
> 
> Decode exception handling was added by commit
> 
>   6ea6e84309ca ("KVM: x86: inject exceptions produced by x86_decode_insn")
> 
> but it was dead code even then.  The patch discussion[1] even point out that
> it was dead code, i.e. the change probably should have been reverted.
> 
> 
> Peng Hao and Yi Wang later ran into what appears to be the same bug you're
> hitting[2][3], and even had patches temporarily queued[4][5], but the
> patches never made it to mainline as they broke kvm-unit-tests.  Fun side
> note, Radim even pointed out[4] the bug fixed by patch 1/3.
> 
> So, the patches look correct, but there's the open question of why the
> hypercall test was failing for Paolo.  I've tried to reproduce the #DF to
> no avail.
> 
> [1] https://lore.kernel.org/patchwork/patch/850077/
> [2] https://lkml.kernel.org/r/1537311828-4547-1-git-send-email-penghao122@xxxxxxxxxxx
> [3] https://lkml.kernel.org/r/20190111133002.GA14852@flask
> [4] https://lkml.kernel.org/r/20190111133002.GA14852@flask
> [5] https://lkml.kernel.org/r/9835d255-dd9a-222b-f4a2-93611175b326@xxxxxxxxxx
> 
> >  			if (emulation_type & EMULTYPE_SKIP)
> >  				return EMULATE_FAIL;
> >  			return handle_emulation_failure(vcpu, emulation_type);
> > -- 
> > 2.1.4
> >