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Date:   Thu, 27 Oct 2022 16:56:10 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Michal Luczaj <mhal@rbox.co>,
        David Woodhouse <dwmw2@infradead.org>
Subject: Re: [PATCH v2 03/16] KVM: x86: Always use non-compat
 vcpu_runstate_info size for gfn=>pfn cache
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 <afad5f40-03ef-1380-9bfe-03bbaaed47a9@redhat.com>
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On Thu, Oct 27, 2022, Sean Christopherson wrote:
> On Thu, Oct 27, 2022, Paolo Bonzini wrote:
> > On 10/13/22 23:12, Sean Christopherson wrote:
> > > Always use the size of Xen's non-compat vcpu_runstate_info struct when
> > > checking that the GPA+size doesn't cross a page boundary.  Conceptually,
> > > using the current mode is more correct, but KVM isn't consistent with
> > > itself as kvm_xen_vcpu_set_attr() unconditionally uses the "full" size
> > > when activating the cache.  More importantly, prior to the introduction
> > > of the gfn_to_pfn_cache, KVM _always_ used the full size, i.e. allowing
> > > the guest (userspace?) to use a poorly aligned GPA in 32-bit mode but not
> > > 64-bit mode is more of a bug than a feature, and fixing the bug doesn't
> > > break KVM's historical ABI.
> > 
> > I'd rather not introduce additional restrictions in KVM,
> 
> But KVM already has this restriction.  "struct vcpu_info" is always checked for
> the non-compat size, and as above, "struct vcpu_runstate_info" is checked for the
> non-compat size during its initialization.

Ah, I forgot those are the same size:

		BUILD_BUG_ON(sizeof(struct vcpu_info) !=
			     sizeof(struct compat_vcpu_info));