xen/hypercall: fix hypercall fallback code for very old hypervisors

While copying the argument structures in HYPERVISOR_event_channel_op()
and HYPERVISOR_physdev_op() into the local variable is sufficiently
safe even if the actual structure is smaller than the container one,
copying back eventual output values the same way isn't: This may
collide with on-stack variables (particularly "rc") which may change
between the first and second memcpy() (i.e. the second memcpy() could
discard that change).

Move the fallback code into out-of-line functions, and handle all of
the operations known by this old a hypervisor individually: Some don't
require copying back anything at all, and for the rest use the
individual argument structures' sizes rather than the container's.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
[v2: Reduce #define/#undef usage in HYPERVISOR_physdev_op_compat().]
[v3: Fix compile errors when modules use said hypercalls]
[v4: Add xen_ prefix to the HYPERCALL_..]
[v5: Alter the name and only EXPORT_SYMBOL_GPL one of them]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/include/asm/xen/hypercall.h

@@ -359,18 +359,14 @@ HYPERVISOR_update_va_mapping(unsigned long va, pte_t new_val,
 		return _hypercall4(int, update_va_mapping, va,
 				   new_val.pte, new_val.pte >> 32, flags);
 }
+extern int __must_check xen_event_channel_op_compat(int, void *);
 
 static inline int
 HYPERVISOR_event_channel_op(int cmd, void *arg)
 {
 	int rc = _hypercall2(int, event_channel_op, cmd, arg);
-	if (unlikely(rc == -ENOSYS)) {
-		struct evtchn_op op;
-		op.cmd = cmd;
-		memcpy(&op.u, arg, sizeof(op.u));
-		rc = _hypercall1(int, event_channel_op_compat, &op);
-		memcpy(arg, &op.u, sizeof(op.u));
-	}
+	if (unlikely(rc == -ENOSYS))
+		rc = xen_event_channel_op_compat(cmd, arg);
 	return rc;
 }
 
@@ -386,17 +382,14 @@ HYPERVISOR_console_io(int cmd, int count, char *str)
 	return _hypercall3(int, console_io, cmd, count, str);
 }
 
+extern int __must_check HYPERVISOR_physdev_op_compat(int, void *);
+
 static inline int
 HYPERVISOR_physdev_op(int cmd, void *arg)
 {
 	int rc = _hypercall2(int, physdev_op, cmd, arg);
-	if (unlikely(rc == -ENOSYS)) {
-		struct physdev_op op;
-		op.cmd = cmd;
-		memcpy(&op.u, arg, sizeof(op.u));
-		rc = _hypercall1(int, physdev_op_compat, &op);
-		memcpy(arg, &op.u, sizeof(op.u));
-	}
+	if (unlikely(rc == -ENOSYS))
+		rc = HYPERVISOR_physdev_op_compat(cmd, arg);
 	return rc;
 }
 

drivers/xen/Makefile

@@ -2,7 +2,7 @@ ifneq ($(CONFIG_ARM),y)
 obj-y	+= manage.o balloon.o
 obj-$(CONFIG_HOTPLUG_CPU)		+= cpu_hotplug.o
 endif
-obj-y	+= grant-table.o features.o events.o
+obj-y	+= grant-table.o features.o events.o fallback.o
 obj-y	+= xenbus/
 
 nostackp := $(call cc-option, -fno-stack-protector)

drivers/xen/fallback.c

@@ -0,0 +1,80 @@
+#include <linux/kernel.h>
+#include <linux/string.h>
+#include <linux/bug.h>
+#include <linux/export.h>
+#include <asm/hypervisor.h>
+#include <asm/xen/hypercall.h>
+
+int xen_event_channel_op_compat(int cmd, void *arg)
+{
+	struct evtchn_op op;
+	int rc;
+
+	op.cmd = cmd;
+	memcpy(&op.u, arg, sizeof(op.u));
+	rc = _hypercall1(int, event_channel_op_compat, &op);
+
+	switch (cmd) {
+	case EVTCHNOP_close:
+	case EVTCHNOP_send:
+	case EVTCHNOP_bind_vcpu:
+	case EVTCHNOP_unmask:
+		/* no output */
+		break;
+
+#define COPY_BACK(eop) \
+	case EVTCHNOP_##eop: \
+		memcpy(arg, &op.u.eop, sizeof(op.u.eop)); \
+		break
+
+	COPY_BACK(bind_interdomain);
+	COPY_BACK(bind_virq);
+	COPY_BACK(bind_pirq);
+	COPY_BACK(status);
+	COPY_BACK(alloc_unbound);
+	COPY_BACK(bind_ipi);
+#undef COPY_BACK
+
+	default:
+		WARN_ON(rc != -ENOSYS);
+		break;
+	}
+
+	return rc;
+}
+EXPORT_SYMBOL_GPL(xen_event_channel_op_compat);
+
+int HYPERVISOR_physdev_op_compat(int cmd, void *arg)
+{
+	struct physdev_op op;
+	int rc;
+
+	op.cmd = cmd;
+	memcpy(&op.u, arg, sizeof(op.u));
+	rc = _hypercall1(int, physdev_op_compat, &op);
+
+	switch (cmd) {
+	case PHYSDEVOP_IRQ_UNMASK_NOTIFY:
+	case PHYSDEVOP_set_iopl:
+	case PHYSDEVOP_set_iobitmap:
+	case PHYSDEVOP_apic_write:
+		/* no output */
+		break;
+
+#define COPY_BACK(pop, fld) \
+	case PHYSDEVOP_##pop: \
+		memcpy(arg, &op.u.fld, sizeof(op.u.fld)); \
+		break
+
+	COPY_BACK(irq_status_query, irq_status_query);
+	COPY_BACK(apic_read, apic_op);
+	COPY_BACK(ASSIGN_VECTOR, irq_op);
+#undef COPY_BACK
+
+	default:
+		WARN_ON(rc != -ENOSYS);
+		break;
+	}
+
+	return rc;
+}