[PATCH 02/11] KVM: x86/mmu: Treat invalid shadow pages as obsolete

From: Sean Christopherson
Date: Thu Sep 12 2019 - 22:46:30 EST

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@xxxxxxxxx>
arch/x86/kvm/mmu.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 5ac5e3f50f92..373e6f052f9f 100644
--- 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,