Re: [PATCH 09/54] KVM: x86/mmu: Unconditionally zap unsync SPs when creating >4k SP at GFN

From: Paolo Bonzini
Date: Wed Jun 23 2021 - 10:36:58 EST

On 22/06/21 19:56, Sean Christopherson wrote:
When creating a new upper-level shadow page, zap unsync shadow pages at
the same target gfn instead of attempting to sync the pages. This fixes
a bug where an unsync shadow page could be sync'd with an incompatible
context, e.g. wrong smm, is_guest, etc... flags. In practice, the bug is
relatively benign as sync_page() is all but guaranteed to fail its check
that the guest's desired gfn (for the to-be-sync'd page) matches the
current gfn associated with the shadow page. I.e. kvm_sync_page() would
end up zapping the page anyways.

Alternatively, __kvm_sync_page() could be modified to explicitly verify
the mmu_role of the unsync shadow page is compatible with the current MMU
context. But, except for this specific case, __kvm_sync_page() is called
iff the page is compatible, e.g. the transient sync in kvm_mmu_get_page()
requires an exact role match, and the call from kvm_sync_mmu_roots() is
only synchronizing shadow pages from the current MMU (which better be
compatible or KVM has problems). And as described above, attempting to
sync shadow pages when creating an upper-level shadow page is unlikely
to succeed, e.g. zero successful syncs were observed when running Linux
guests despite over a million attempts.

One issue, this WARN_ON may now trigger:


due to a kvm_mmu_prepare_zap_page that could have happened on an earlier iteration of the for_each_valid_sp. Before your change, __kvm_sync_page would be called always before kvm_sync_pages could add anything to invalid_list.