drivers: hv: Turn off write permission on the hypercall page

The hypercall page only needs to be executable but currently it is setup to
be writable as well. Fix the issue.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Cc: <stable@vger.kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Reported-by: Stephen Hemminger <stephen@networkplumber.org>
Tested-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/hyperv/hv_init.c

@@ -125,7 +125,7 @@ void hyperv_init(void)
 	guest_id = generate_guest_id(0, LINUX_VERSION_CODE, 0);
 	wrmsrl(HV_X64_MSR_GUEST_OS_ID, guest_id);
 
-	hypercall_pg  = __vmalloc(PAGE_SIZE, GFP_KERNEL, PAGE_KERNEL_EXEC);
+	hypercall_pg  = __vmalloc(PAGE_SIZE, GFP_KERNEL, PAGE_KERNEL_RX);
 	if (hypercall_pg == NULL) {
 		wrmsrl(HV_X64_MSR_GUEST_OS_ID, 0);
 		return;