Re: [PATCH] kvm/x86/mmu: use the correct inherited permissions to get shadow page

[Sean Christopherson <seanjc@google.com>]

On Fri, Nov 20, 2020, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
> 
> Commit 41074d07c78b ("KVM: MMU: Fix inherited permissions for emulated
> guest pte updates") said role.access is common access permissions for
> all ptes in this shadow page, which is the inherited permissions from
> the parent ptes.
> 
> But the commit did not enforce this definition when kvm_mmu_get_page()
> is called in FNAME(fetch). Rather, it uses a random (last level pte's
> combined) access permissions.

I wouldn't say it's random, the issue is specifically that all shadow pages end
up using the combined set of permissions of the entire walk, as opposed to the
only combined permissions of its parents.

> And the permissions won't be checked again in next FNAME(fetch) since the
> spte is present. It might fail to meet guest's expectation when guest sets up
> spaghetti pagetables.

Can you provide details on the exact failure scenario?  It would be very helpful
for documentation and understanding.  I can see how using the full combined
permissions will cause weirdness for upper level SPs in kvm_mmu_get_page(), but
I'm struggling to connect the dots to understand how that will cause incorrect
behavior for the guest.  AFAICT, outside of the SP cache, KVM only consumes
role.access for the final/last SP.