Re: [PATCH] fanotify: Make sure to check event_len when copying

[Amir Goldstein]

On Wed, Dec 5, 2018 at 1:44 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> As a precaution, make sure we check event_len when copying to userspace.
> Based on old feedback: https://lkml.kernel.org/r/542D9FE5.3010009@xxxxxx
>

This precaution serves the new fanotify_event_info_fid format well:
https://marc.info/?l=linux-fsdevel&m=154375073318106&w=2
but also conflicts with my series.

I can weave your patch into my series and add a similar sanity check in
copy_fid_to_user() or if Jan applies your patch and has more comments
on my v4 series, I'll rebase on top of that.

Thanks,
Amir.

> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  fs/notify/fanotify/fanotify_user.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index e03be5071362..d9484a0ac6b3 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -206,7 +206,7 @@ static int process_access_response(struct fsnotify_group *group,
>
>  static ssize_t copy_event_to_user(struct fsnotify_group *group,
>                                   struct fsnotify_event *event,
> -                                 char __user *buf)
> +                                 char __user *buf, size_t count)
>  {
>         struct fanotify_event_metadata fanotify_event_metadata;
>         struct file *f;
> @@ -220,6 +220,12 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>
>         fd = fanotify_event_metadata.fd;
>         ret = -EFAULT;
> +       /*
> +        * Sanity check copy size in case get_one_event() and
> +        * fill_event_metadata() event_len sizes ever get out of sync.
> +        */
> +       if (WARN_ON_ONCE(fanotify_event_metadata.event_len > count))
> +               goto out_close_fd;
>         if (copy_to_user(buf, &fanotify_event_metadata,
>                          fanotify_event_metadata.event_len))
>                 goto out_close_fd;
> @@ -295,7 +301,7 @@ static ssize_t fanotify_read(struct file *file, char __user *buf,
>                         continue;
>                 }
>
> -               ret = copy_event_to_user(group, kevent, buf);
> +               ret = copy_event_to_user(group, kevent, buf, count);
>                 if (unlikely(ret == -EOPENSTALE)) {
>                         /*
>                          * We cannot report events with stale fd so drop it.
> --
> 2.17.1
>
>
> --
> Kees Cook