linux-kernel - Re: [PATCH v2 03/10] KVM: arm/arm64: vgic-its: Improve error reporting on device table save

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Date:   Fri, 13 Oct 2017 19:56:53 +0200
From:   Christoffer Dall <cdall@...aro.org>
To:     Auger Eric <eric.auger@...hat.com>
Cc:     eric.auger.pro@...il.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, marc.zyngier@....com,
        peter.maydell@...aro.org, andre.przywara@....com,
        wanghaibin.wang@...wei.com, wu.wubin@...wei.com
Subject: Re: [PATCH v2 03/10] KVM: arm/arm64: vgic-its: Improve error
 reporting on device table save

On Fri, Oct 13, 2017 at 04:22:25PM +0200, Auger Eric wrote:
> Hi,
> 
> On 13/10/2017 15:16, Christoffer Dall wrote:
> > On Wed, Sep 27, 2017 at 03:28:33PM +0200, Eric Auger wrote:
> >> At the moment the device table save() returns -EINVAL if
> >> vgic_its_check_id() fails to return the gpa of the entry
> >> associated to the device/collection id. Let vgic_its_check_id()
> >> return an int instead of a bool and return a more precised
> >> error value:
> >> - EINVAL in case the id is out of range
> >> - EFAULT if the gpa is not provisionned or is not valid
> >>
> > 
> > This is just to ease debugging, yes?
> 
> I understood user-space should be able to discriminate between bad guest
> programming and values corrupted by the userspace (regs for instance).
> In first case QEMU should not abort. In latter case it should abort.

So what is userspace supposed to do in the first case?

> 
> In vgic_its_check_id we are checking the L1 entry validity bit and in
> case it is invalid we can't compute the GPA of the entry. I was thinking
> we should return -EFAULT in that case. But maybe returning EFAULT in
> case the BASER<n> address is not reachable also is wrong because that
> may be caused by the userspace writing a wrong value. Sigh ...
> 

I think if either userspace or the guest programmed something that
cannot be traversed, then you just don't save/restore the ITS properly,
because it's broken anyway, so I don't think we need to replicate the
*same broken state* at the destination.

Maybe I'm missing part of the picture here.

Thanks,
-Christoffer