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KVM: x86/mmu: Add lockdep assert to enforce safe usage of kvm_unmap_gfn_range()

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Add a lockdep assertion in kvm_unmap_gfn_range() to ensure that either
mmu_invalidate_in_progress is elevated, or that the range is being zapped
due to memslot removal (loosely detected by slots_lock being held).
Zapping SPTEs without mmu_invalidate_{in_progress,seq} protection is unsafe
as KVM's page fault path snapshots state before acquiring mmu_lock, and
thus can create SPTEs with stale information if vCPUs aren't forced to
retry faults (due to seeing an in-progress or past MMU invalidation).

Memslot removal is a special case, as the memslot is retrieved outside of
mmu_invalidate_seq, i.e. doesn't use the "standard" protections, and
instead relies on SRCU synchronization to ensure any in-flight page faults
are fully resolved before zapping SPTEs.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20241009192345.1148353-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2024-10-09 12:23:44 -07:00 committed by Paolo Bonzini
parent 58a20a9435
commit 28cf497881
1 changed files with 11 additions and 0 deletions
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11
arch/x86/kvm/mmu/mmu.c
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@ -1556,6 +1556,17 @@ bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
{
bool flush = false;
/*
* To prevent races with vCPUs faulting in a gfn using stale data,
* zapping a gfn range must be protected by mmu_invalidate_in_progress
* (and mmu_invalidate_seq). The only exception is memslot deletion;
* in that case, SRCU synchronization ensures that SPTEs are zapped
* after all vCPUs have unlocked SRCU, guaranteeing that vCPUs see the
* invalid slot.
*/
lockdep_assert_once(kvm->mmu_invalidate_in_progress ||
lockdep_is_held(&kvm->slots_lock));
if (kvm_memslots_have_rmaps(kvm))
flush = __kvm_rmap_zap_gfn_range(kvm, range->slot,
range->start, range->end,