x86/hvm: Fix altp2m_op hypercall continuations

c/s 9383de210 "x86/altp2m: support for setting restrictions for an array of
pages" introduced this logic, but do_hvm_op() was already capable of handling
-ERESTART correctly.

More problematic however is a continuation from compat_altp2m_op().  The arg
written back into register state points into the hypercall XLAT area, not at
the original parameter passed by the guest.  It may be truncated by the
vmentry, but definitely won't be correct on the next invocation.

Delete the hypercall_create_continuation() call, and return -ERESTART, which
will cause the compat case to start working correctly.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 8228577ad1ba9f4b49370b76c90b75fb9243ee2f
master date: 2019-04-09 19:34:41 +0100

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 8adbb61b57c5ebe3e6ec8258ce118e6e395a4790..209ed976585bf88a3e65803ce5caef5fe47b7d34 100644 (file)
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -4713,12 +4713,10 @@ static int do_altp2m_op(
         if ( rc > 0 )
         {
             a.u.set_mem_access_multi.opaque = rc;
+            rc = -ERESTART;
             if ( __copy_field_to_guest(guest_handle_cast(arg, xen_hvm_altp2m_op_t),
                                        &a, u.set_mem_access_multi.opaque) )
                 rc = -EFAULT;
-            else
-                rc = hypercall_create_continuation(__HYPERVISOR_hvm_op, "lh",
-                                                   HVMOP_altp2m, arg);
         }
         break;
 
@@ -4837,14 +4835,8 @@ static int compat_altp2m_op(
     switch ( a.cmd )
     {
     case HVMOP_altp2m_set_mem_access_multi:
-        /*
-         * The return code can be positive only if it is the return value
-         * of hypercall_create_continuation. In this case, the opaque value
-         * must be copied back to the guest.
-         */
-        if ( rc > 0 )
+        if ( rc == -ERESTART )
         {
-            ASSERT(rc == __HYPERVISOR_hvm_op);
             a.u.set_mem_access_multi.opaque =
                 nat.altp2m_op->u.set_mem_access_multi.opaque;
             if ( __copy_field_to_guest(guest_handle_cast(arg,