[PATCH V2 01/10] KVM: X86: Fix missed remote tlb flush in rmap_write_protect()

Lai Jiangshan
2021-09-18T00:56:43Z

When kvm->tlbs_dirty > 0, some rmaps might have been deleted
without flushing tlb remotely after kvm_sync_page(). If @gfn
was writable before and it's rmaps was deleted in kvm_sync_page(),
and if the tlb entry is still in a remote running VCPU, the @gfn
is not safely protected.

To fix the problem, kvm_sync_page() does the remote flush when
needed to avoid the problem.

Fixes: a4ee1ca4a36e ("KVM: MMU: delay flush all tlbs on sync_page path")
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
Changed from V1:
force remote flush timely instead of increasing tlbs_dirty.

arch/x86/kvm/mmu/paging_tmpl.h | 23 ++---------------------
1 file changed, 2 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 72f358613786..5962d4f8a72e 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -1038,14 +1038,6 @@ static gpa_t FNAME(gva_to_gpa_nested)(struct kvm_vcpu *vcpu, gpa_t vaddr,
* Using the cached information from sp->gfns is safe because:
* - The spte has a reference to the struct page, so the pfn for a given gfn
* can't change unless all sptes pointing to it are nuked first.
- *
- * Note:
- * We should flush all tlbs if spte is dropped even though guest is
- * responsible for it. Since if we don't, kvm_mmu_notifier_invalidate_page
- * and kvm_mmu_notifier_invalidate_range_start detect the mapping page isn't
- * used by guest then tlbs are not flushed, so guest is allowed to access the
- * freed pages.
- * And we increase kvm->tlbs_dirty to delay tlbs flush in this case.
*/
static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
{
@@ -1098,13 +1090,7 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
return 0;

if (FNAME(prefetch_invalid_gpte)(vcpu, sp, &sp->spt[i], gpte)) {
- /*
- * Update spte before increasing tlbs_dirty to make
- * sure no tlb flush is lost after spte is zapped; see
- * the comments in kvm_flush_remote_tlbs().
- */
- smp_wmb();
- vcpu->kvm->tlbs_dirty++;
+ set_spte_ret |= SET_SPTE_NEED_REMOTE_TLB_FLUSH;
continue;
}

@@ -1119,12 +1105,7 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)

if (gfn != sp->gfns[i]) {
drop_spte(vcpu->kvm, &sp->spt[i]);
- /*
- * The same as above where we are doing
- * prefetch_invalid_gpte().
- */
- smp_wmb();
- vcpu->kvm->tlbs_dirty++;
+ set_spte_ret |= SET_SPTE_NEED_REMOTE_TLB_FLUSH;
continue;
}

--
2.19.1.6.gb485710b