KVM: x86: MMU: Make mmu_set_spte() return emulate value

mmu_set_spte()'s code is based on the assumption that the emulate
parameter has a valid pointer value if set_spte() returns true and
write_fault is not zero.  In other cases, emulate may be NULL, so a
NULL-check is needed.

Stop passing emulate pointer and make mmu_set_spte() return the emulate
value instead to clean up this complex interface.  Prefetch functions
can just throw away the return value.
Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

CVE-2018-12207

(cherry picked from commit 029499b4)
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

arch/x86/kvm/mmu.c

@@ -2580,13 +2580,13 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 	return ret;
 }
 
-static void mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
-			 unsigned pte_access, int write_fault, int *emulate,
-			 int level, gfn_t gfn, pfn_t pfn, bool speculative,
-			 bool host_writable)
+static bool mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep, unsigned pte_access,
+			 int write_fault, int level, gfn_t gfn, pfn_t pfn,
+			 bool speculative, bool host_writable)
 {
 	int was_rmapped = 0;
 	int rmap_count;
+	bool emulate = false;
 
 	pgprintk("%s: spte %llx write_fault %d gfn %llx\n", __func__,
 		 *sptep, write_fault, gfn);
@@ -2616,12 +2616,12 @@ static void mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 	if (set_spte(vcpu, sptep, pte_access, level, gfn, pfn, speculative,
 	      true, host_writable)) {
 		if (write_fault)
-			*emulate = 1;
+			emulate = true;
 		kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
 	}
 
-	if (unlikely(is_mmio_spte(*sptep) && emulate))
-		*emulate = 1;
+	if (unlikely(is_mmio_spte(*sptep)))
+		emulate = true;
 
 	pgprintk("%s: setting spte %llx\n", __func__, *sptep);
 	pgprintk("instantiating %s PTE (%s) at %llx (%llx) addr %p\n",
@@ -2640,6 +2640,8 @@ static void mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 	}
 
 	kvm_release_pfn_clean(pfn);
+
+	return emulate;
 }
 
 static pfn_t pte_prefetch_gfn_to_pfn(struct kvm_vcpu *vcpu, gfn_t gfn,
@@ -2674,9 +2676,8 @@ static int direct_pte_prefetch_many(struct kvm_vcpu *vcpu,
 		return -1;
 
 	for (i = 0; i < ret; i++, gfn++, start++)
-		mmu_set_spte(vcpu, start, access, 0, NULL,
-			     sp->role.level, gfn, page_to_pfn(pages[i]),
-			     true, true);
+		mmu_set_spte(vcpu, start, access, 0, sp->role.level, gfn,
+			     page_to_pfn(pages[i]), true, true);
 
 	return 0;
 }
@@ -2738,9 +2739,9 @@ static int __direct_map(struct kvm_vcpu *vcpu, gpa_t v, int write,
 
 	for_each_shadow_entry(vcpu, (u64)gfn << PAGE_SHIFT, iterator) {
 		if (iterator.level == level) {
-			mmu_set_spte(vcpu, iterator.sptep, ACC_ALL,
-				     write, &emulate, level, gfn, pfn,
-				     prefault, map_writable);
+			emulate = mmu_set_spte(vcpu, iterator.sptep, ACC_ALL,
+					       write, level, gfn, pfn, prefault,
+					       map_writable);
 			direct_pte_prefetch(vcpu, iterator.sptep);
 			++vcpu->stat.pf_fixed;
 			break;

arch/x86/kvm/paging_tmpl.h

@@ -475,8 +475,8 @@ FNAME(prefetch_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 	 * we call mmu_set_spte() with host_writable = true because
 	 * pte_prefetch_gfn_to_pfn always gets a writable pfn.
 	 */
-	mmu_set_spte(vcpu, spte, pte_access, 0, NULL, PT_PAGE_TABLE_LEVEL,
-		     gfn, pfn, true, true);
+	mmu_set_spte(vcpu, spte, pte_access, 0, PT_PAGE_TABLE_LEVEL, gfn, pfn,
+		     true, true);
 
 	return true;
 }
@@ -556,7 +556,7 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, gva_t addr,
 	struct kvm_mmu_page *sp = NULL;
 	struct kvm_shadow_walk_iterator it;
 	unsigned direct_access, access = gw->pt_access;
-	int top_level, emulate = 0;
+	int top_level, emulate;
 
 	direct_access = gw->pte_access;
 
@@ -622,8 +622,8 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, gva_t addr,
 	}
 
 	clear_sp_write_flooding_count(it.sptep);
-	mmu_set_spte(vcpu, it.sptep, gw->pte_access, write_fault, &emulate,
-		     it.level, gw->gfn, pfn, prefault, map_writable);
+	emulate = mmu_set_spte(vcpu, it.sptep, gw->pte_access, write_fault,
+			       it.level, gw->gfn, pfn, prefault, map_writable);
 	FNAME(pte_prefetch)(vcpu, gw, it.sptep);
 
 	return emulate;