KVM: x86/mmu: WARN on any attempt to atomically update REMOVED SPTE
Disallow calling tdp_mmu_set_spte_atomic() with a REMOVED "old" SPTE. This solves a conundrum introduced by commit 3255530a ("KVM: x86/mmu: Automatically update iter->old_spte if cmpxchg fails"); if the helper doesn't update old_spte in the REMOVED case, then theoretically the caller could get stuck in an infinite loop as it will fail indefinitely on the REMOVED SPTE. E.g. until recently, clear_dirty_gfn_range() didn't check for a present SPTE and would have spun until getting rescheduled. In practice, only the page fault path should "create" a new SPTE, all other paths should only operate on existing, a.k.a. shadow present, SPTEs. Now that the page fault path pre-checks for a REMOVED SPTE in all cases, require all other paths to indirectly pre-check by verifying the target SPTE is a shadow-present SPTE. Note, this does not guarantee the actual SPTE isn't REMOVED, nor is that scenario disallowed. The invariant is only that the caller mustn't invoke tdp_mmu_set_spte_atomic() if the SPTE was REMOVED when last observed by the caller. Cc: David Matlack <dmatlack@google.com> Signed-off-by: Sean Christopherson <seanjc@google.com> Message-Id: <20220226001546.360188-25-seanjc@google.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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