Commit fac026da authored Sep 12, 2019 by Sean Christopherson Committed by Paolo Bonzini Sep 24, 2019

KVM: x86/mmu: Treat invalid shadow pages as obsolete

Treat invalid shadow pages as obsolete to fix a bug where an obsolete
and invalid page with a non-zero root count could become non-obsolete
due to mmu_valid_gen wrapping. The bug is largely theoretical with the
current code base, as an unsigned long will effectively never wrap on
64-bit KVM, and userspace would have to deliberately stall a vCPU in
order to keep an obsolete invalid page on the active list while
simultaneously modifying memslots billions of times to trigger a wrap.

The obvious alternative is to use a 64-bit value for mmu_valid_gen,
but it's actually desirable to go in the opposite direction, i.e. using
a smaller 8-bit value to reduce KVM's memory footprint by 8 bytes per
shadow page, and relying on proper treatment of invalid pages instead of
preventing the generation from wrapping.

Note, "Fixes" points at a commit that was at one point reverted, but has
since been restored.

Fixes: 5304b8d3 ("KVM: MMU: fast invalidate all pages")
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

parent d0f5a86a

arch/x86/kvm/mmu.c

+3 −2

Original line number	Diff line number	Diff line
		@@ -2252,7 +2252,7 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
		#define for_each_valid_sp(_kvm, _sp, _gfn) \
		hlist_for_each_entry(_sp, \
		&(_kvm)->arch.mmu_page_hash[kvm_page_table_hashfn(_gfn)], hash_link) \
		if (is_obsolete_sp((_kvm), (_sp)) \|\| (_sp)->role.invalid) { \
		if (is_obsolete_sp((_kvm), (_sp))) { \
		} else

		#define for_each_gfn_indirect_valid_sp(_kvm, _sp, _gfn) \
		@@ -2311,7 +2311,8 @@ static void mmu_audit_disable(void) { }

		static bool is_obsolete_sp(struct kvm kvm, struct kvm_mmu_page sp)
		{
		return unlikely(sp->mmu_valid_gen != kvm->arch.mmu_valid_gen);
		return sp->role.invalid \|\|
		unlikely(sp->mmu_valid_gen != kvm->arch.mmu_valid_gen);
		}

		static bool kvm_sync_page(struct kvm_vcpu vcpu, struct kvm_mmu_page sp,

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