Re: [PATCH 6.1.y 2/2] KVM: x86: Check for invalid/obsolete root *after* making MMU pages available

On Thu, Jul 30, 2026, Sasha Levin wrote:
> From: Sean Christopherson <[email protected]>
> 
> [ Upstream commit 2abd5287f08319fa35764566b15c6e22cb1068db ]
> 
> 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.
> 
> Reported-by: Hyunwoo Kim <[email protected]>
> Fixes: f95eec9bed76 ("KVM: x86/mmu: Don't put invalid SPs back on the list of active pages")
> Cc: [email protected]
> Signed-off-by: Sean Christopherson <[email protected]>
> Signed-off-by: Paolo Bonzini <[email protected]>
> Signed-off-by: Sasha Levin <[email protected]>
> ---

Acked-by: Sean Christopherson <[email protected]>