[PATCH] xen/vcpu: ensure no write when runstate handler is NULL

[Roger Pau Monne <roger@xxxxxxxxxxxxxx>]

Both the normal and the compat versions of
VCPUOP_register_runstate_memory_area allow registering a NULL handler as a
way to signal deregistration of the previous area, and
update_runstate_area() will skip the update when the handler is NULL.

However when handling the VCPUOP_register_runstate_memory_area hypercall
itself there's an unconditional attempt to update the newly registered
runstate area, without taking into account whether it points to NULL.  Skip
such update if the user-provided handle is NULL.

Signed-off-by: Roger Pau Monné <roger@xxxxxxxxxxxxxx>
---
 xen/arch/x86/x86_64/domain.c | 4 ++++
 xen/common/domain.c          | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/xen/arch/x86/x86_64/domain.c b/xen/arch/x86/x86_64/domain.c
index a02d4f569ee5..e4d49cbbfeb6 100644
--- a/xen/arch/x86/x86_64/domain.c
+++ b/xen/arch/x86/x86_64/domain.c
@@ -58,6 +58,10 @@ compat_vcpu_op(int cmd, unsigned int vcpuid, 
XEN_GUEST_HANDLE_PARAM(void) arg)
         rc = 0;
         guest_from_compat_handle(v->runstate_guest.compat, area.addr.h);
 
+        /* NULL handle is deregistering the address. */
+        if ( compat_handle_is_null(area.addr.h) )
+            break;
+
         if ( v == current )
         {
             XLAT_vcpu_runstate_info(&info, &v->runstate);
diff --git a/xen/common/domain.c b/xen/common/domain.c
index 6b52713518da..30a17b8b7edb 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -2330,6 +2330,10 @@ long common_vcpu_op(int cmd, struct vcpu *v, 
XEN_GUEST_HANDLE_PARAM(void) arg)
         rc = 0;
         runstate_guest(v) = area.addr.h;
 
+        /* NULL handle is deregistering the address. */
+        if ( guest_handle_is_null(area.addr.h) )
+            break;
+
         if ( v == current )
         {
             __copy_to_guest(runstate_guest(v), &v->runstate, 1);
-- 
2.55.0