[PATCH v2 3/5] x86/xlat: fix UB pointer arithmetic in COMPAT_ARG_XLAT_VIRT_BASE

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

UBSAN complains with:

UBSAN: Undefined behaviour in common/compat/memory.c:90:9
pointer operation overflowed ffff820080000000 to 0000020080000000
[...]
Xen call trace:
   [<ffff82d040303882>] R common/ubsan/ubsan.c#ubsan_epilogue+0xa/0xc0
   [<ffff82d040304cc3>] F 
lib/xxhash64.c#__ubsan_handle_pointer_overflow+0xcb/0x100
   [<ffff82d0402a6359>] F lib/xxhash64.c#compat_memory_op+0xf1/0x4d20
   [<ffff82d04041545d>] F lib/xxhash64.c#hvm_memory_op+0x55/0xe0
   [<ffff82d040416280>] F lib/xxhash64.c#hvm_hypercall+0xae8/0x21b0
   [<ffff82d0403b25ca>] F lib/xxhash64.c#svm_vmexit_handler+0x1252/0x2450
   [<ffff82d0402049c0>] F lib/xxhash64.c#svm_stgi_label+0x5/0x15

Adjust the calculations in COMPAT_ARG_XLAT_VIRT_BASE to subtract from the
per-domain area to obtain the mirrored linear address in the 4th slot,
instead of overflowing the per-domain linear address.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - New in this version.
---
 xen/arch/x86/include/asm/x86_64/uaccess.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/include/asm/x86_64/uaccess.h 
b/xen/arch/x86/include/asm/x86_64/uaccess.h
index c6fa3fd381bc..f933707e109f 100644
--- a/xen/arch/x86/include/asm/x86_64/uaccess.h
+++ b/xen/arch/x86/include/asm/x86_64/uaccess.h
@@ -9,9 +9,9 @@
  * a secondary mapping installed, which needs to be used for such accesses in
  * the PV case, and will also be used for HVM to avoid extra conditionals.
  */
-#define COMPAT_ARG_XLAT_VIRT_BASE ((void *)ARG_XLAT_START(current) + \
-                                   (PERDOMAIN_ALT_VIRT_START - \
-                                    PERDOMAIN_VIRT_START))
+#define COMPAT_ARG_XLAT_VIRT_BASE ((void *)ARG_XLAT_START(current) - \
+                                   (PERDOMAIN_VIRT_START - \
+                                    PERDOMAIN_ALT_VIRT_START))
 #define COMPAT_ARG_XLAT_SIZE      (2*PAGE_SIZE)
 struct vcpu;
 int setup_compat_arg_xlat(struct vcpu *v);
-- 
2.48.1