mm: prevent KASAN false positives in kmemleak

When kmemleak dumps contents of leaked objects it reads whole objects
regardless of user-requested size.  This upsets KASAN.  Disable KASAN
checks around object dump.

Link: http://lkml.kernel.org/r/1466617631-68387-1-git-send-email-dvyukov@google.com
Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/kmemleak.c

@@ -307,8 +307,10 @@ static void hex_dump_object(struct seq_file *seq,
 	len = min_t(size_t, object->size, HEX_MAX_LINES * HEX_ROW_SIZE);
 
 	seq_printf(seq, "  hex dump (first %zu bytes):\n", len);
+	kasan_disable_current();
 	seq_hex_dump(seq, "    ", DUMP_PREFIX_NONE, HEX_ROW_SIZE,
 		     HEX_GROUP_SIZE, ptr, len, HEX_ASCII);
+	kasan_enable_current();
 }
 
 /*