Subject: [PATCH] ARM/KVM: inject data abort on unhandled memory access

From: [email-masked] (Christoffer Dall)
Date: Tue, 10 Dec 2013 16:38:30 -0800

On Thu, Dec 05, 2013 at 06:24:13PM +0000, Marc Zyngier wrote:
> Hi Andre,
> 
> On 05/12/13 15:10, Andre Przywara wrote:
> > If a KVM guest accesses memory that is outside its memory map (so no
> > MMIO and no RAM), KVM will return -ENOSYS to userland, causing QEMU
> > to do an abort() and kill the whole guest. This happens while
> > executing dmidecode on ARM, which mmaps /dev/mem and scans the first
> > Megabyte of memory for a DMI BIOS signature (sic!).
> 
> Arghhh. And of course, I expect they do that using instructions we can't
> use for IOs.
> 
> Bummer.
> 
> > Of course this is silly, but in any case crashing the whole guest
> > does not seems appropriate.
> > So lets mimic native hardware's behavior in this case and inject a
> > Data Abort exception into the guest. In the previous case this will
> > crash dmidecode with SIGSEGV, but keeps the guest alive.
> > 
> > I am not sure if this too coarse grained, but I just wanted to start
> > discussion on this.
> > 
> > Signed-off-by: Andre Przywara <[email-masked]>
> > ---
> >  arch/arm/kvm/mmio.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm/kvm/mmio.c b/arch/arm/kvm/mmio.c
> > index 4cb5a93..04a105e 100644
> > --- a/arch/arm/kvm/mmio.c
> > +++ b/arch/arm/kvm/mmio.c
> > @@ -183,7 +183,8 @@ int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run *run,
> >  			return ret;
> >  	} else {
> >  		kvm_err("load/store instruction decoding not implemented\n");
> > -		return -ENOSYS;
> > +		kvm_inject_dabt(vcpu, kvm_vcpu_get_hfar(vcpu));
> > +		return 1;
> >  	}
> >  
> >  	rt = vcpu->arch.mmio_decode.rt;
> > 
> 
> I agree that killing the whole VM is not the nicest thing in the world.
> How about:
> - keeping some form of warning
> - rate-limit it so we don't flood the host
> - inject the data-abort
> 
> That should give us a saner behaviour (I agree with you that the current
> one is not very good), and yet annoy the luser enough so that they
> either fix their software or start merging the emulation code...
> 
I think this is quite reasonable - the guest does something completely
valid, but happens to hit an unimplemented part of KVM.  It's really up
to user space to deal with this accordingly.  Injecting a data abort is
wrong, IMHO, because it is not remotely what hardware would do.

The proper fix is to add the necessary instruction emulation...

-Christoffer