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: 5304b8d37c2a5 ("KVM: MMU: fast invalidate all pages")
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 84e1a9c..151c4b9 100644 (file)
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -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,