mm/usercopy.c: no check page span for stack objects

It is easy to trigger this with CONFIG_HARDENED_USERCOPY_PAGESPAN=y,

  usercopy: Kernel memory overwrite attempt detected to spans multiple pages (offset 0, size 23)!
  kernel BUG at mm/usercopy.c:102!

For example,

print_worker_info
char name[WQ_NAME_LEN] = { };
char desc[WORKER_DESC_LEN] = { };
  probe_kernel_read(name, wq->name, sizeof(name) - 1);
  probe_kernel_read(desc, worker->desc, sizeof(desc) - 1);
    __copy_from_user_inatomic
      check_object_size
        check_heap_object
          check_page_span

This is because on-stack variables could cross PAGE_SIZE boundary, and
failed this check,

if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) ==
	   ((unsigned long)end & (unsigned long)PAGE_MASK)))

ptr = FFFF889007D7EFF8
end = FFFF889007D7F00E

Hence, fix it by checking if it is a stack object first.

[keescook@chromium.org: improve comments after reorder]
  Link: http://lkml.kernel.org/r/20190103165151.GA32845@beast
Link: http://lkml.kernel.org/r/20181231030254.99441-1-cai@lca.pwSigned-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/usercopy.c

@@ -247,7 +247,8 @@ static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks);
 /*
  * Validates that the given object is:
  * - not bogus address
- * - known-safe heap or stack object
+ * - fully contained by stack (or stack frame, when available)
+ * - fully within SLAB object (or object whitelist area, when available)
  * - not in kernel text
  */
 void __check_object_size(const void *ptr, unsigned long n, bool to_user)
@@ -262,9 +263,6 @@ void __check_object_size(const void *ptr, unsigned long n, bool to_user)
 	/* Check for invalid addresses. */
 	check_bogus_address((const unsigned long)ptr, n, to_user);
 
-	/* Check for bad heap object. */
-	check_heap_object(ptr, n, to_user);
-
 	/* Check for bad stack object. */
 	switch (check_stack_object(ptr, n)) {
 	case NOT_STACK:
@@ -282,6 +280,9 @@ void __check_object_size(const void *ptr, unsigned long n, bool to_user)
 		usercopy_abort("process stack", NULL, to_user, 0, n);
 	}
 
+	/* Check for bad heap object. */
+	check_heap_object(ptr, n, to_user);
+
 	/* Check for object in kernel to avoid text exposure. */
 	check_kernel_text_object((const unsigned long)ptr, n, to_user);
 }