KVM: x86: #PF error-code on R/W operations is wrong

When emulating an instruction that reads the destination memory operand (i.e.,
instructions without the Mov flag in the emulator), the operand is first read.
If a page-fault is detected in this phase, the error-code which would be
delivered to the VM does not indicate that the access that caused the exception
is a write one. This does not conform with real hardware, and may cause the VM
to enter the page-fault handler twice for no reason (once for read, once for
write).

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

3 files changed, 17 insertions, 13 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index cb19d05af3cd..97a5dd0222c8 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -160,6 +160,18 @@ enum {
 #define DR7_FIXED_1     0x00000400
 #define DR7_VOLATILE    0xffff2bff
 
+#define PFERR_PRESENT_BIT 0
+#define PFERR_WRITE_BIT 1
+#define PFERR_USER_BIT 2
+#define PFERR_RSVD_BIT 3
+#define PFERR_FETCH_BIT 4
+
+#define PFERR_PRESENT_MASK (1U << PFERR_PRESENT_BIT)
+#define PFERR_WRITE_MASK (1U << PFERR_WRITE_BIT)
+#define PFERR_USER_MASK (1U << PFERR_USER_BIT)
+#define PFERR_RSVD_MASK (1U << PFERR_RSVD_BIT)
+#define PFERR_FETCH_MASK (1U << PFERR_FETCH_BIT)
+
 /* apic attention bits */
 #define KVM_APIC_CHECK_VAPIC    0
 /*
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index d949287ed010..ef23c1e5fa9f 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -4909,8 +4909,12 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
                 /* optimisation - avoid slow emulated read if Mov */
                 rc = segmented_read(ctxt, ctxt->dst.addr.mem,
                                    &ctxt->dst.val, ctxt->dst.bytes);
-                if (rc != X86EMUL_CONTINUE)
+                if (rc != X86EMUL_CONTINUE) {
+                        if (rc == X86EMUL_PROPAGATE_FAULT &&
+                            ctxt->exception.vector == PF_VECTOR)
+                                ctxt->exception.error_code |= PFERR_WRITE_MASK;
                         goto done;
+                }
         }
         ctxt->dst.orig_val = ctxt->dst.val;
 
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index a7f9a121690d..c7d65637c851 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -44,18 +44,6 @@
 #define PT_DIRECTORY_LEVEL 2
 #define PT_PAGE_TABLE_LEVEL 1
 
-#define PFERR_PRESENT_BIT 0
-#define PFERR_WRITE_BIT 1
-#define PFERR_USER_BIT 2
-#define PFERR_RSVD_BIT 3
-#define PFERR_FETCH_BIT 4
-
-#define PFERR_PRESENT_MASK (1U << PFERR_PRESENT_BIT)
-#define PFERR_WRITE_MASK (1U << PFERR_WRITE_BIT)
-#define PFERR_USER_MASK (1U << PFERR_USER_BIT)
-#define PFERR_RSVD_MASK (1U << PFERR_RSVD_BIT)
-#define PFERR_FETCH_MASK (1U << PFERR_FETCH_BIT)
-
 static inline u64 rsvd_bits(int s, int e)
 {
         return ((1ULL << (e - s + 1)) - 1) << s;