KVM: X86: Synchronize the shadow pagetable before link it

If gpte is changed from non-present to present, the guest doesn't need
to flush tlb per SDM.  So the host must synchronze sp before
link it.  Otherwise the guest might use a wrong mapping.

For example: the guest first changes a level-1 pagetable, and then
links its parent to a new place where the original gpte is non-present.
Finally the guest can access the remapped area without flushing
the tlb.  The guest's behavior should be allowed per SDM, but the host
kvm mmu makes it wrong.

Fixes: 4731d4c7a077 ("KVM: MMU: out of sync shadow core")
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210918005636.3675-3-jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 2d7e611..1a64ba5 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2027,8 +2027,8 @@ static void mmu_pages_clear_parents(struct mmu_page_path *parents)
        } while (!sp->unsync_children);
 }
 
-static void mmu_sync_children(struct kvm_vcpu *vcpu,
-                             struct kvm_mmu_page *parent)
+static int mmu_sync_children(struct kvm_vcpu *vcpu,
+                            struct kvm_mmu_page *parent, bool can_yield)
 {
        int i;
        struct kvm_mmu_page *sp;
@@ -2055,12 +2055,18 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu,
                }
                if (need_resched() || rwlock_needbreak(&vcpu->kvm->mmu_lock)) {
                        kvm_mmu_flush_or_zap(vcpu, &invalid_list, false, flush);
+                       if (!can_yield) {
+                               kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
+                               return -EINTR;
+                       }
+
                        cond_resched_rwlock_write(&vcpu->kvm->mmu_lock);
                        flush = false;
                }
        }
 
        kvm_mmu_flush_or_zap(vcpu, &invalid_list, false, flush);
+       return 0;
 }
 
 static void __clear_sp_write_flooding_count(struct kvm_mmu_page *sp)
@@ -2146,9 +2152,6 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
                        kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
                }
 
-               if (sp->unsync_children)
-                       kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
-
                __clear_sp_write_flooding_count(sp);
 
 trace_get_page:
@@ -3684,7 +3687,7 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
                write_lock(&vcpu->kvm->mmu_lock);
                kvm_mmu_audit(vcpu, AUDIT_PRE_SYNC);
 
-               mmu_sync_children(vcpu, sp);
+               mmu_sync_children(vcpu, sp, true);
 
                kvm_mmu_audit(vcpu, AUDIT_POST_SYNC);
                write_unlock(&vcpu->kvm->mmu_lock);
@@ -3700,7 +3703,7 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
                if (IS_VALID_PAE_ROOT(root)) {
                        root &= PT64_BASE_ADDR_MASK;
                        sp = to_shadow_page(root);
-                       mmu_sync_children(vcpu, sp);
+                       mmu_sync_children(vcpu, sp, true);
                }
        }
 

diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index efce0a9..913d52a 100644 (file)
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -707,8 +707,27 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, gpa_t addr,
                if (!is_shadow_present_pte(*it.sptep)) {
                        table_gfn = gw->table_gfn[it.level - 2];
                        access = gw->pt_access[it.level - 2];
-                       sp = kvm_mmu_get_page(vcpu, table_gfn, addr, it.level-1,
-                                             false, access);
+                       sp = kvm_mmu_get_page(vcpu, table_gfn, addr,
+                                             it.level-1, false, access);
+                       /*
+                        * We must synchronize the pagetable before linking it
+                        * because the guest doesn't need to flush tlb when
+                        * the gpte is changed from non-present to present.
+                        * Otherwise, the guest may use the wrong mapping.
+                        *
+                        * For PG_LEVEL_4K, kvm_mmu_get_page() has already
+                        * synchronized it transiently via kvm_sync_page().
+                        *
+                        * For higher level pagetable, we synchronize it via
+                        * the slower mmu_sync_children().  If it needs to
+                        * break, some progress has been made; return
+                        * RET_PF_RETRY and retry on the next #PF.
+                        * KVM_REQ_MMU_SYNC is not necessary but it
+                        * expedites the process.
+                        */
+                       if (sp->unsync_children &&
+                           mmu_sync_children(vcpu, sp, false))
+                               return RET_PF_RETRY;
                }
 
                /*