KVM: x86: improve reexecute_instruction

The current reexecute_instruction can not well detect the failed instruction
emulation. It allows guest to retry all the instructions except it accesses
on error pfn

For example, some cases are nested-write-protect - if the page we want to
write is used as PDE but it chains to itself. Under this case, we should
stop the emulation and report the case to userspace
Reviewed-by: NGleb Natapov <gleb@redhat.com>
Signed-off-by: NXiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -497,6 +497,13 @@ struct kvm_vcpu_arch {
 		u64 msr_val;
 		struct gfn_to_hva_cache data;
 	} pv_eoi;
+
+	/*
+	 * Indicate whether the access faults on its page table in guest
+	 * which is set when fix page fault and used to detect unhandeable
+	 * instruction.
+	 */
+	bool write_fault_to_shadow_pgtable;
 };
 
 struct kvm_lpage_info {

arch/x86/kvm/paging_tmpl.h

@@ -497,26 +497,34 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, gva_t addr,
  * created when kvm establishes shadow page table that stop kvm using large
  * page size. Do it early can avoid unnecessary #PF and emulation.
  *
+ * @write_fault_to_shadow_pgtable will return true if the fault gfn is
+ * currently used as its page table.
+ *
  * Note: the PDPT page table is not checked for PAE-32 bit guest. It is ok
  * since the PDPT is always shadowed, that means, we can not use large page
  * size to map the gfn which is used as PDPT.
  */
 static bool
 FNAME(is_self_change_mapping)(struct kvm_vcpu *vcpu,
-			      struct guest_walker *walker, int user_fault)
+			      struct guest_walker *walker, int user_fault,
+			      bool *write_fault_to_shadow_pgtable)
 {
 	int level;
 	gfn_t mask = ~(KVM_PAGES_PER_HPAGE(walker->level) - 1);
+	bool self_changed = false;
 
 	if (!(walker->pte_access & ACC_WRITE_MASK ||
 	      (!is_write_protection(vcpu) && !user_fault)))
 		return false;
 
-	for (level = walker->level; level <= walker->max_level; level++)
-		if (!((walker->gfn ^ walker->table_gfn[level - 1]) & mask))
-			return true;
+	for (level = walker->level; level <= walker->max_level; level++) {
+		gfn_t gfn = walker->gfn ^ walker->table_gfn[level - 1];
 
-	return false;
+		self_changed |= !(gfn & mask);
+		*write_fault_to_shadow_pgtable |= !gfn;
+	}
+
+	return self_changed;
 }
 
 /*
@@ -544,7 +552,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
 	int level = PT_PAGE_TABLE_LEVEL;
 	int force_pt_level;
 	unsigned long mmu_seq;
-	bool map_writable;
+	bool map_writable, is_self_change_mapping;
 
 	pgprintk("%s: addr %lx err %x\n", __func__, addr, error_code);
 
@@ -572,9 +580,14 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
 		return 0;
 	}
 
+	vcpu->arch.write_fault_to_shadow_pgtable = false;
+
+	is_self_change_mapping = FNAME(is_self_change_mapping)(vcpu,
+	      &walker, user_fault, &vcpu->arch.write_fault_to_shadow_pgtable);
+
 	if (walker.level >= PT_DIRECTORY_LEVEL)
 		force_pt_level = mapping_level_dirty_bitmap(vcpu, walker.gfn)
-		   || FNAME(is_self_change_mapping)(vcpu, &walker, user_fault);
+		   || is_self_change_mapping;
 	else
 		force_pt_level = 1;
 	if (!force_pt_level) {

arch/x86/kvm/x86.c

@@ -4751,7 +4751,8 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu)
 	return r;
 }
 
-static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t cr2)
+static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t cr2,
+				  bool write_fault_to_shadow_pgtable)
 {
 	gpa_t gpa = cr2;
 	pfn_t pfn;
@@ -4808,7 +4809,13 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t cr2)
 	 * guest to let CPU execute the instruction.
 	 */
 	kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
-	return true;
+
+	/*
+	 * If the access faults on its page table, it can not
+	 * be fixed by unprotecting shadow page and it should
+	 * be reported to userspace.
+	 */
+	return !write_fault_to_shadow_pgtable;
 }
 
 static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
@@ -4867,7 +4874,13 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
 	int r;
 	struct x86_emulate_ctxt *ctxt = &vcpu->arch.emulate_ctxt;
 	bool writeback = true;
+	bool write_fault_to_spt = vcpu->arch.write_fault_to_shadow_pgtable;
 
+	/*
+	 * Clear write_fault_to_shadow_pgtable here to ensure it is
+	 * never reused.
+	 */
+	vcpu->arch.write_fault_to_shadow_pgtable = false;
 	kvm_clear_exception_queue(vcpu);
 
 	if (!(emulation_type & EMULTYPE_NO_DECODE)) {
@@ -4886,7 +4899,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
 		if (r != EMULATION_OK)  {
 			if (emulation_type & EMULTYPE_TRAP_UD)
 				return EMULATE_FAIL;
-			if (reexecute_instruction(vcpu, cr2))
+			if (reexecute_instruction(vcpu, cr2,
+						  write_fault_to_spt))
 				return EMULATE_DONE;
 			if (emulation_type & EMULTYPE_SKIP)
 				return EMULATE_FAIL;
@@ -4916,7 +4930,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
 		return EMULATE_DONE;
 
 	if (r == EMULATION_FAILED) {
-		if (reexecute_instruction(vcpu, cr2))
+		if (reexecute_instruction(vcpu, cr2, write_fault_to_spt))
 			return EMULATE_DONE;
 
 		return handle_emulation_failure(vcpu);