ARM: disable grant table v2

It was never expected to work, the implementation is incomplete.

As a side effect, it also prevents guests from triggering a
"BUG_ON(page_get_owner(pg) != d)" in gnttab_unpopulate_status_frames().

This is XSA-268.

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 9a5c16a3e7)

docs/misc/xen-command-line.markdown

@@ -933,6 +933,8 @@ version are 1 and 2.
 use of grant table v2 without transitive grants is an ABI breakage from the
 guests point of view.
 
+The usage of gnttab v2 is not security supported on ARM platforms.
+
 ### gnttab\_max\_frames
 > `= <integer>`
 

xen/common/grant_table.c

@@ -97,7 +97,11 @@ static unsigned int __read_mostly max_maptrack_frames =
                                                DEFAULT_MAX_MAPTRACK_FRAMES;
 integer_runtime_param("gnttab_max_maptrack_frames", max_maptrack_frames);
 
-static unsigned int __read_mostly opt_gnttab_max_version = 2;
+#ifndef GNTTAB_MAX_VERSION
+#define GNTTAB_MAX_VERSION 2
+#endif
+
+static unsigned int __read_mostly opt_gnttab_max_version = GNTTAB_MAX_VERSION;
 static bool __read_mostly opt_transitive_grants = true;
 
 static int __init parse_gnttab(const char *s)

xen/include/asm-arm/grant_table.h

@@ -7,6 +7,7 @@
 #include <xen/sched.h>
 
 #define INITIAL_NR_GRANT_FRAMES 1U
+#define GNTTAB_MAX_VERSION 1
 
 struct grant_table_arch {
     gfn_t *shared_gfn;