Re: [PATCH] KVM: x86: Check for invalid/obsolete root *after* making MMU pages available

On Sat, Aug 08, 2026, Sergey Senozhatsky wrote:
> On (26/08/08 15:22), Sergey Senozhatsky wrote:
> > From: Sean Christopherson <[email protected]>
> > 
> > Check for a "stale" page fault, i.e. for an invalid and/or obsolete root,
> > after making MMU pages available for the shadow MMU.  If reclaiming shadow
> > pages zaps an in-use root, i.e. marks it invalid, then KVM will attempt to
> > map memory into an invalid root.  On its own, populating an invalid root is
> > "fine", but because child shadow pages inherit their parent's role, any
> > children created during the map/fetch will be created as invalid pages,
> > thus violating KVM's invariant that invalid pages are never on the list of
> > active MMU pages.
> > 
> > Note, the underlying flaw has existed since KVM first started tracking
> > invalid roots in 2008 (commit 2e53d63acba7, "KVM: MMU: ignore zapped root
> > pagetables"), but the true badness only came along in 2020 (Linux 5.9)
> > with the invariant that invalid shadow pages can't be on the list of
> > active pages.
> > 
> > Note #2, inheriting role.invalid when creating child shadow pages is also
> > far from ideal; that flaw will be addressed separately.
> > 
> > Conflicts:
> > 	arch/x86/kvm/mmu/mmu.c
> > 	arch/x86/kvm/mmu/paging_tmpl.h
> 
> Sean, can you please take a look if conflict resolution was done
> correctly?

What kernel is this for?