[PATCH v2 4/5] x86/shadow: fix UB pointer arithmetic in sh_mfn_is_a_page_table()

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

UBSAN complains with:

UBSAN: Undefined behaviour in arch/x86/mm/shadow/private.h:515:30
pointer operation overflowed ffff82e000000000 to ffff82dfffffffe0
[...]
Xen call trace:
   [<ffff82d040303882>] R common/ubsan/ubsan.c#ubsan_epilogue+0xa/0xc0
   [<ffff82d040304cc3>] F 
lib/xxhash64.c#__ubsan_handle_pointer_overflow+0xcb/0x100
   [<ffff82d040471c5d>] F 
arch/x86/mm/shadow/guest_2.c#sh_page_fault__guest_2+0x1e350
   [<ffff82d0403b216b>] F lib/xxhash64.c#svm_vmexit_handler+0xdf3/0x2450
   [<ffff82d0402049c0>] F lib/xxhash64.c#svm_stgi_label+0x5/0x15

Fix by moving the call to mfn_to_page() after the check of whether the
passed gmfn is valid.  This avoid the call to mfn_to_page() with an
INVALID_MFN parameter.

While there make the page local variable const, it's not modified by the
function.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - New in this version.
---
 xen/arch/x86/mm/shadow/private.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/mm/shadow/private.h b/xen/arch/x86/mm/shadow/private.h
index a5fc3a7676eb..cef9dbef2e77 100644
--- a/xen/arch/x86/mm/shadow/private.h
+++ b/xen/arch/x86/mm/shadow/private.h
@@ -512,13 +512,14 @@ static inline unsigned long __backpointer(const struct 
page_info *sp)
 static inline int
 sh_mfn_is_a_page_table(mfn_t gmfn)
 {
-    struct page_info *page = mfn_to_page(gmfn);
+    const struct page_info *page;
     struct domain *owner;
     unsigned long type_info;
 
     if ( !mfn_valid(gmfn) )
         return 0;
 
+    page = mfn_to_page(gmfn);
     owner = page_get_owner(page);
     if ( owner && shadow_mode_refcounts(owner)
          && (page->count_info & PGC_shadowed_pt) )
-- 
2.48.1