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

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (26/08/10 07:08), Sean Christopherson wrote:
> On Sat, Aug 08, 2026, Sergey Senozhatsky wrote:
> > On (26/08/08 15:22), Sergey Senozhatsky wrote:
> > > From: Sean Christopherson <seanjc@google.com>
> > > 
> > > 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?

Oh... good point.  I forgot to specify [PATCH 6.1.y]