[PATCH v5 5/5] KVM: x86: improve reexecute_instruction

[Xiao Guangrong]

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

Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/kvm_host.h |    7 +++++++
 arch/x86/kvm/paging_tmpl.h      |   27 ++++++++++++++++++++-------
 arch/x86/kvm/x86.c              |    8 +++++++-
 3 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index c431b33..d6ab8d2 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -502,6 +502,13 @@ 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 67b390d..df50560 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 6f13e03..2957012 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4810,7 +4810,13 @@ 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 !vcpu->arch.write_fault_to_shadow_pgtable;
 }

 static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
-- 
1.7.7.6

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