On Mon, May 16, 2022, David Matlack wrote: > On Fri, May 13, 2022 at 5:56 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > On Fri, May 13, 2022, David Matlack wrote: > > > On Fri, May 13, 2022 at 12:50 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > > > > > Drop SPTEs whose new protections will yield a RWX=0 SPTE, i.e. a SPTE > > > > that is marked shadow-present but is not-present in the page tables. If > > > > EPT with execute-only support is in use by L1, KVM can create a RWX=0 > > > > SPTE can be created for an EPTE if the upper level combined permissions > > > > are R (or RW) and the leaf EPTE is changed from R (or RW) to X. > > > > > > For some reason I found this sentence hard to read. > > > > Heh, probably because "KVM can create a RWX=0 SPTE can be created" is nonsensical. > > I botched a late edit to the changelog... > > > > > What about this: > > > > > > When shadowing EPT and NX HugePages is enabled, if the guest changes > > > > This doesn' thave anything to do with NX HugePages, it's an execute-only specific > > bug where L1 can create a gPTE that is !READABLE but is considered PRESENT because > > it is EXECUTABLE. If the upper level protections are R or RW, the resulting > > protections for the entire translation are RWX=0. All of sync_page()'s existing > > checks filter out only !PRESENT gPTE, because without execute-only, all upper > > levels are guaranteed to be at least READABLE. > > I see what you mean, thanks. > > And I also recall now you mentioned (off-list) that the NX HugePage > scenario isn't possible because KVM does not let huge pages go unsync. Yep. The other thing that's semi-relevant and I've mentioned off-list at least once is that our (Google's) old kernel has a different NX HugePage implementation that _can_ result in RWX=0 SPTEs. Unlike upstream, the internal NX HugePage implementation shatters a huge page _after_ installing said huge page, whereas upstream demotes the huge page before it's installed. If shattering fails on huge page that L1 created a huge page with just X permissions, KVM is left with a RWX=0 huge page.
