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

3 files changed, 45 insertions, 11 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index f75e1feb6ec5..77d56a4ba89c 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/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 {
diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 3d1a35237dbf..ca69dcccbe31 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -497,26 +497,34 @@ out_gpte_changed:
  * 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];
+
+                self_changed |= !(gfn & mask);
+                *write_fault_to_shadow_pgtable |= !gfn;
+        }
 
-        return false;
+        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) {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6f9cab071eca..e00dd0515a84 100644
--- a/arch/x86/kvm/x86.c
+++ b/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 @@ restart:
                 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);