Re: [PATCH] mm: fix filler_t callback type mismatch with readpage

[Kees Cook <keescook@chromium.org>]

On Tue, Apr 30, 2019 at 2:47 PM Sami Tolvanen <samitolvanen@google.com> wrote:
>
> Casting mapping->a_ops->readpage to filler_t causes an indirect call
> type mismatch with Control-Flow Integrity checking. This change fixes
> the mismatch in read_cache_page_gfp and read_mapping_page by adding a
> stub callback function with the correct type.
>
> As the kernel only has a couple of instances of read_cache_page(s)
> being called with a callback function that doesn't accept struct file*
> as the first parameter, Android kernels have previously fixed this by
> changing filler_t to int (*filler_t)(struct file *, struct page *):
>
>   https://android-review.googlesource.com/c/kernel/common/+/671260
>
> While this approach did fix most of the issues, the few remaining
> cases where unrelated private data are passed to the callback become
> rather awkward. Keeping filler_t unchanged and using a stub function
> for readpage instead solves this problem.
>
> Cc: Kees Cook <keescook@chromium.org>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  include/linux/pagemap.h | 22 +++++++++++++++++++---
>  mm/filemap.c            |  7 +++++--
>  2 files changed, 24 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index bcf909d0de5f8..e5652a5ba1584 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -383,11 +383,27 @@ extern struct page * read_cache_page_gfp(struct address_space *mapping,
>  extern int read_cache_pages(struct address_space *mapping,
>                 struct list_head *pages, filler_t *filler, void *data);
>
> +struct file_filler_data {
> +       int (*filler)(struct file *, struct page *);
> +       struct file *filp;
> +};
> +
> +static inline int __file_filler(void *data, struct page *page)
> +{
> +       struct file_filler_data *ffd = (struct file_filler_data *)data;
> +
> +       return ffd->filler(ffd->filp, page);
> +}
> +
>  static inline struct page *read_mapping_page(struct address_space *mapping,
> -                               pgoff_t index, void *data)
> +                               pgoff_t index, struct file *filp)
>  {
> -       filler_t *filler = (filler_t *)mapping->a_ops->readpage;
> -       return read_cache_page(mapping, index, filler, data);
> +       struct file_filler_data data = {
> +               .filler = mapping->a_ops->readpage,
> +               .filp   = filp
> +       };
> +
> +       return read_cache_page(mapping, index, __file_filler, &data);
>  }
>
>  /*
> diff --git a/mm/filemap.c b/mm/filemap.c
> index d78f577baef2a..6cc41c25ca3bf 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2977,9 +2977,12 @@ struct page *read_cache_page_gfp(struct address_space *mapping,
>                                 pgoff_t index,
>                                 gfp_t gfp)
>  {
> -       filler_t *filler = (filler_t *)mapping->a_ops->readpage;
> +       struct file_filler_data data = {
> +               .filler = mapping->a_ops->readpage,
> +               .filp   = NULL
> +       };
>
> -       return do_read_cache_page(mapping, index, filler, NULL, gfp);
> +       return do_read_cache_page(mapping, index, __file_filler, &data, gfp);
>  }
>  EXPORT_SYMBOL(read_cache_page_gfp);
>
> --
> 2.21.0.593.g511ec345e18-goog
>


-- 
Kees Cook