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

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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: 5304b8d37c ("KVM: MMU: fast invalidate all pages")
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2019-09-12 19:46:03 -07:00 committed by Paolo Bonzini
parent d0f5a86a34
commit fac026dac0
1 changed files with 3 additions and 2 deletions
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5
arch/x86/kvm/mmu.c
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@ -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,