[PATCH 2/3] x86/percpu: Force INVALID_PERCPU_AREA into the non-canonical address region

[Andrew Cooper <andrew.cooper3@citrix.com>]

This causes accidental uses of per_cpu() on a pcpu with an INVALID_PERCPU_AREA
to result in a #GF for attempting to access the middle of the non-canonical
virtual address region.

This is preferable to the current behaviour, where incorrect use of per_cpu()
will result in an effective NULL structure dereference which has security
implication in the context of PV guests.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Keir Fraser <keir@xen.org>
CC: Jan Beulich <JBeulich@suse.com>
CC: Tim Deegan <tim@xen.org>
---
 xen/arch/x86/percpu.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/percpu.c b/xen/arch/x86/percpu.c
index e545024..1c1dad9 100644
--- a/xen/arch/x86/percpu.c
+++ b/xen/arch/x86/percpu.c
@@ -6,7 +6,14 @@
 #include <xen/rcupdate.h>
 
 unsigned long __per_cpu_offset[NR_CPUS];
-#define INVALID_PERCPU_AREA (-(long)__per_cpu_start)
+
+/*
+ * Force uses of per_cpu() with an invalid area to attempt to access the
+ * middle of the non-canonical address space resulting in a #GP, rather than a
+ * possible #PF at (NULL + a little) which has security implications in the
+ * context of PV guests.
+ */
+#define INVALID_PERCPU_AREA (0x8000000000000000L - (long)__per_cpu_start)
 #define PERCPU_ORDER (get_order_from_bytes(__per_cpu_data_end-__per_cpu_start))
 
 void __init percpu_init_areas(void)
-- 
1.7.10.4