KVM: x86: Handle errors when RIP is set during far jumps

Far jmp/call/ret may fault while loading a new RIP.  Currently KVM does not
handle this case, and may result in failed vm-entry once the assignment is
done.  The tricky part of doing so is that loading the new CS affects the
VMCS/VMCB state, so if we fail during loading the new RIP, we are left in
unconsistent state.  Therefore, this patch saves on 64-bit the old CS
descriptor and restores it if loading RIP failed.

This fixes CVE-2014-3647.

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

arch/x86/kvm/emulate.c

@@ -1443,7 +1443,9 @@ static int write_segment_descriptor(struct x86_emulate_ctxt *ctxt,
 
 /* Does not support long mode */
 static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
-				     u16 selector, int seg, u8 cpl, bool in_task_switch)
+				     u16 selector, int seg, u8 cpl,
+				     bool in_task_switch,
+				     struct desc_struct *desc)
 {
 	struct desc_struct seg_desc, old_desc;
 	u8 dpl, rpl;
@@ -1584,6 +1586,8 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
 	}
 load:
 	ctxt->ops->set_segment(ctxt, selector, &seg_desc, base3, seg);
+	if (desc)
+		*desc = seg_desc;
 	return X86EMUL_CONTINUE;
 exception:
 	return emulate_exception(ctxt, err_vec, err_code, true);
@@ -1593,7 +1597,7 @@ static int load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
 				   u16 selector, int seg)
 {
 	u8 cpl = ctxt->ops->cpl(ctxt);
-	return __load_segment_descriptor(ctxt, selector, seg, cpl, false);
+	return __load_segment_descriptor(ctxt, selector, seg, cpl, false, NULL);
 }
 
 static void write_register_operand(struct operand *op)
@@ -1987,17 +1991,31 @@ static int em_iret(struct x86_emulate_ctxt *ctxt)
 static int em_jmp_far(struct x86_emulate_ctxt *ctxt)
 {
 	int rc;
-	unsigned short sel;
+	unsigned short sel, old_sel;
+	struct desc_struct old_desc, new_desc;
+	const struct x86_emulate_ops *ops = ctxt->ops;
+	u8 cpl = ctxt->ops->cpl(ctxt);
+
+	/* Assignment of RIP may only fail in 64-bit mode */
+	if (ctxt->mode == X86EMUL_MODE_PROT64)
+		ops->get_segment(ctxt, &old_sel, &old_desc, NULL,
+				 VCPU_SREG_CS);
 
 	memcpy(&sel, ctxt->src.valptr + ctxt->op_bytes, 2);
 
-	rc = load_segment_descriptor(ctxt, sel, VCPU_SREG_CS);
+	rc = __load_segment_descriptor(ctxt, sel, VCPU_SREG_CS, cpl, false,
+				       &new_desc);
 	if (rc != X86EMUL_CONTINUE)
 		return rc;
 
-	ctxt->_eip = 0;
-	memcpy(&ctxt->_eip, ctxt->src.valptr, ctxt->op_bytes);
-	return X86EMUL_CONTINUE;
+	rc = assign_eip_far(ctxt, ctxt->src.val, new_desc.l);
+	if (rc != X86EMUL_CONTINUE) {
+		WARN_ON(!ctxt->mode != X86EMUL_MODE_PROT64);
+		/* assigning eip failed; restore the old cs */
+		ops->set_segment(ctxt, old_sel, &old_desc, 0, VCPU_SREG_CS);
+		return rc;
+	}
+	return rc;
 }
 
 static int em_grp45(struct x86_emulate_ctxt *ctxt)
@@ -2064,21 +2082,34 @@ static int em_ret(struct x86_emulate_ctxt *ctxt)
 static int em_ret_far(struct x86_emulate_ctxt *ctxt)
 {
 	int rc;
-	unsigned long cs;
+	unsigned long eip, cs;
+	u16 old_cs;
 	int cpl = ctxt->ops->cpl(ctxt);
+	struct desc_struct old_desc, new_desc;
+	const struct x86_emulate_ops *ops = ctxt->ops;
+
+	if (ctxt->mode == X86EMUL_MODE_PROT64)
+		ops->get_segment(ctxt, &old_cs, &old_desc, NULL,
+				 VCPU_SREG_CS);
 
-	rc = emulate_pop(ctxt, &ctxt->_eip, ctxt->op_bytes);
+	rc = emulate_pop(ctxt, &eip, ctxt->op_bytes);
 	if (rc != X86EMUL_CONTINUE)
 		return rc;
-	if (ctxt->op_bytes == 4)
-		ctxt->_eip = (u32)ctxt->_eip;
 	rc = emulate_pop(ctxt, &cs, ctxt->op_bytes);
 	if (rc != X86EMUL_CONTINUE)
 		return rc;
 	/* Outer-privilege level return is not implemented */
 	if (ctxt->mode >= X86EMUL_MODE_PROT16 && (cs & 3) > cpl)
 		return X86EMUL_UNHANDLEABLE;
-	rc = load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS);
+	rc = __load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS, 0, false,
+				       &new_desc);
+	if (rc != X86EMUL_CONTINUE)
+		return rc;
+	rc = assign_eip_far(ctxt, eip, new_desc.l);
+	if (rc != X86EMUL_CONTINUE) {
+		WARN_ON(!ctxt->mode != X86EMUL_MODE_PROT64);
+		ops->set_segment(ctxt, old_cs, &old_desc, 0, VCPU_SREG_CS);
+	}
 	return rc;
 }
 
@@ -2505,19 +2536,24 @@ static int load_state_from_tss16(struct x86_emulate_ctxt *ctxt,
 	 * Now load segment descriptors. If fault happens at this stage
 	 * it is handled in a context of new task
 	 */
-	ret = __load_segment_descriptor(ctxt, tss->ldt, VCPU_SREG_LDTR, cpl, true);
+	ret = __load_segment_descriptor(ctxt, tss->ldt, VCPU_SREG_LDTR, cpl,
+					true, NULL);
 	if (ret != X86EMUL_CONTINUE)
 		return ret;
-	ret = __load_segment_descriptor(ctxt, tss->es, VCPU_SREG_ES, cpl, true);
+	ret = __load_segment_descriptor(ctxt, tss->es, VCPU_SREG_ES, cpl,
+					true, NULL);
 	if (ret != X86EMUL_CONTINUE)
 		return ret;
-	ret = __load_segment_descriptor(ctxt, tss->cs, VCPU_SREG_CS, cpl, true);
+	ret = __load_segment_descriptor(ctxt, tss->cs, VCPU_SREG_CS, cpl,
+					true, NULL);
 	if (ret != X86EMUL_CONTINUE)
 		return ret;
-	ret = __load_segment_descriptor(ctxt, tss->ss, VCPU_SREG_SS, cpl, true);
+	ret = __load_segment_descriptor(ctxt, tss->ss, VCPU_SREG_SS, cpl,
+					true, NULL);
 	if (ret != X86EMUL_CONTINUE)
 		return ret;
-	ret = __load_segment_descriptor(ctxt, tss->ds, VCPU_SREG_DS, cpl, true);
+	ret = __load_segment_descriptor(ctxt, tss->ds, VCPU_SREG_DS, cpl,
+					true, NULL);
 	if (ret != X86EMUL_CONTINUE)
 		return ret;
 
@@ -2642,25 +2678,32 @@ static int load_state_from_tss32(struct x86_emulate_ctxt *ctxt,
 	 * Now load segment descriptors. If fault happenes at this stage
 	 * it is handled in a context of new task
 	 */
-	ret = __load_segment_descriptor(ctxt, tss->ldt_selector, VCPU_SREG_LDTR, cpl, true);
+	ret = __load_segment_descriptor(ctxt, tss->ldt_selector, VCPU_SREG_LDTR,
+					cpl, true, NULL);
 	if (ret != X86EMUL_CONTINUE)
 		return ret;
-	ret = __load_segment_descriptor(ctxt, tss->es, VCPU_SREG_ES, cpl, true);
+	ret = __load_segment_descriptor(ctxt, tss->es, VCPU_SREG_ES, cpl,
+					true, NULL);
 	if (ret != X86EMUL_CONTINUE)
 		return ret;
-	ret = __load_segment_descriptor(ctxt, tss->cs, VCPU_SREG_CS, cpl, true);
+	ret = __load_segment_descriptor(ctxt, tss->cs, VCPU_SREG_CS, cpl,
+					true, NULL);
 	if (ret != X86EMUL_CONTINUE)
 		return ret;
-	ret = __load_segment_descriptor(ctxt, tss->ss, VCPU_SREG_SS, cpl, true);
+	ret = __load_segment_descriptor(ctxt, tss->ss, VCPU_SREG_SS, cpl,
+					true, NULL);
 	if (ret != X86EMUL_CONTINUE)
 		return ret;
-	ret = __load_segment_descriptor(ctxt, tss->ds, VCPU_SREG_DS, cpl, true);
+	ret = __load_segment_descriptor(ctxt, tss->ds, VCPU_SREG_DS, cpl,
+					true, NULL);
 	if (ret != X86EMUL_CONTINUE)
 		return ret;
-	ret = __load_segment_descriptor(ctxt, tss->fs, VCPU_SREG_FS, cpl, true);
+	ret = __load_segment_descriptor(ctxt, tss->fs, VCPU_SREG_FS, cpl,
+					true, NULL);
 	if (ret != X86EMUL_CONTINUE)
 		return ret;
-	ret = __load_segment_descriptor(ctxt, tss->gs, VCPU_SREG_GS, cpl, true);
+	ret = __load_segment_descriptor(ctxt, tss->gs, VCPU_SREG_GS, cpl,
+					true, NULL);
 	if (ret != X86EMUL_CONTINUE)
 		return ret;
 
@@ -2942,24 +2985,39 @@ static int em_call_far(struct x86_emulate_ctxt *ctxt)
 	u16 sel, old_cs;
 	ulong old_eip;
 	int rc;
+	struct desc_struct old_desc, new_desc;
+	const struct x86_emulate_ops *ops = ctxt->ops;
+	int cpl = ctxt->ops->cpl(ctxt);
 
-	old_cs = get_segment_selector(ctxt, VCPU_SREG_CS);
 	old_eip = ctxt->_eip;
+	ops->get_segment(ctxt, &old_cs, &old_desc, NULL, VCPU_SREG_CS);
 
 	memcpy(&sel, ctxt->src.valptr + ctxt->op_bytes, 2);
-	if (load_segment_descriptor(ctxt, sel, VCPU_SREG_CS))
+	rc = __load_segment_descriptor(ctxt, sel, VCPU_SREG_CS, cpl, false,
+				       &new_desc);
+	if (rc != X86EMUL_CONTINUE)
 		return X86EMUL_CONTINUE;
 
-	ctxt->_eip = 0;
-	memcpy(&ctxt->_eip, ctxt->src.valptr, ctxt->op_bytes);
+	rc = assign_eip_far(ctxt, ctxt->src.val, new_desc.l);
+	if (rc != X86EMUL_CONTINUE)
+		goto fail;
 
 	ctxt->src.val = old_cs;
 	rc = em_push(ctxt);
 	if (rc != X86EMUL_CONTINUE)
-		return rc;
+		goto fail;
 
 	ctxt->src.val = old_eip;
-	return em_push(ctxt);
+	rc = em_push(ctxt);
+	/* If we failed, we tainted the memory, but the very least we should
+	   restore cs */
+	if (rc != X86EMUL_CONTINUE)
+		goto fail;
+	return rc;
+fail:
+	ops->set_segment(ctxt, old_cs, &old_desc, 0, VCPU_SREG_CS);
+	return rc;
+
 }
 
 static int em_ret_near_imm(struct x86_emulate_ctxt *ctxt)