KVM: MMU: fix Dirty bit missed if CR0.WP = 0

If the write-fault access is from supervisor and CR0.WP is not set on the
vcpu, kvm will fix it by adjusting pte access - it sets the W bit on pte
and clears U bit. This is the chance that kvm can change pte access from
readonly to writable

Unfortunately, the pte access is the access of 'direct' shadow page table,
means direct sp.role.access = pte_access, then we will create a writable
spte entry on the readonly shadow page table. It will cause Dirty bit is
not tracked when two guest ptes point to the same large page. Note, it
does not have other impact except Dirty bit since cr0.wp is encoded into
sp.role

It can be fixed by adjusting pte access before establishing shadow page
table. Also, after that, no mmu specified code exists in the common function
and drop two parameters in set_spte

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/mmu.c

@@ -2342,8 +2342,7 @@ static int mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
 }
 
 static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
-		    unsigned pte_access, int user_fault,
-		    int write_fault, int level,
+		    unsigned pte_access, int level,
 		    gfn_t gfn, pfn_t pfn, bool speculative,
 		    bool can_unsync, bool host_writable)
 {
@@ -2378,9 +2377,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 
 	spte |= (u64)pfn << PAGE_SHIFT;
 
-	if ((pte_access & ACC_WRITE_MASK)
-	    || (!vcpu->arch.mmu.direct_map && write_fault
-		&& !is_write_protection(vcpu) && !user_fault)) {
+	if (pte_access & ACC_WRITE_MASK) {
 
 		/*
 		 * There are two cases:
@@ -2399,19 +2396,6 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 
 		spte |= PT_WRITABLE_MASK | SPTE_MMU_WRITEABLE;
 
-		if (!vcpu->arch.mmu.direct_map
-		    && !(pte_access & ACC_WRITE_MASK)) {
-			spte &= ~PT_USER_MASK;
-			/*
-			 * If we converted a user page to a kernel page,
-			 * so that the kernel can write to it when cr0.wp=0,
-			 * then we should prevent the kernel from executing it
-			 * if SMEP is enabled.
-			 */
-			if (kvm_read_cr4_bits(vcpu, X86_CR4_SMEP))
-				spte |= PT64_NX_MASK;
-		}
-
 		/*
 		 * Optimization: for pte sync, if spte was writable the hash
 		 * lookup is unnecessary (and expensive). Write protection
@@ -2442,18 +2426,15 @@ done:
 
 static void mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 			 unsigned pt_access, unsigned pte_access,
-			 int user_fault, int write_fault,
-			 int *emulate, int level, gfn_t gfn,
-			 pfn_t pfn, bool speculative,
-			 bool host_writable)
+			 int write_fault, int *emulate, int level, gfn_t gfn,
+			 pfn_t pfn, bool speculative, bool host_writable)
 {
 	int was_rmapped = 0;
 	int rmap_count;
 
-	pgprintk("%s: spte %llx access %x write_fault %d"
-		 " user_fault %d gfn %llx\n",
+	pgprintk("%s: spte %llx access %x write_fault %d gfn %llx\n",
 		 __func__, *sptep, pt_access,
-		 write_fault, user_fault, gfn);
+		 write_fault, gfn);
 
 	if (is_rmap_spte(*sptep)) {
 		/*
@@ -2477,9 +2458,8 @@ static void mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 			was_rmapped = 1;
 	}
 
-	if (set_spte(vcpu, sptep, pte_access, user_fault, write_fault,
-		      level, gfn, pfn, speculative, true,
-		      host_writable)) {
+	if (set_spte(vcpu, sptep, pte_access, level, gfn, pfn, speculative,
+	      true, host_writable)) {
 		if (write_fault)
 			*emulate = 1;
 		kvm_mmu_flush_tlb(vcpu);
@@ -2571,10 +2551,9 @@ 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, ACC_ALL,
-			     access, 0, 0, NULL,
-			     sp->role.level, gfn,
-			     page_to_pfn(pages[i]), true, true);
+		mmu_set_spte(vcpu, start, ACC_ALL, access, 0, NULL,
+			     sp->role.level, gfn, page_to_pfn(pages[i]),
+			     true, true);
 
 	return 0;
 }
@@ -2636,8 +2615,8 @@ static int __direct_map(struct kvm_vcpu *vcpu, gpa_t v, int write,
 			unsigned pte_access = ACC_ALL;
 
 			mmu_set_spte(vcpu, iterator.sptep, ACC_ALL, pte_access,
-				     0, write, &emulate,
-				     level, gfn, pfn, prefault, map_writable);
+				     write, &emulate, level, gfn, pfn,
+				     prefault, map_writable);
 			direct_pte_prefetch(vcpu, iterator.sptep);
 			++vcpu->stat.pf_fixed;
 			break;