Re: [PATCH v2 2/3] fanotify: define struct members to hold response decision context

[Paul Moore]

On Thu, Apr 28, 2022 at 8:45 PM Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
>
> This patch adds 2 structure members to the response returned from user
> space on a permission event. The first field is 16 bits for the context
> type.  The context type will describe what the meaning is of the second
> field. The default is none. The patch defines one additional context
> type which means that the second field is a 32-bit rule number. This
> will allow for the creation of other context types in the future if
> other users of the API identify different needs.  The second field size
> is defined by the context type and can be used to pass along the data
> described by the context.
>
> To support this, there is a macro for user space to check that the data
> being sent is valid. Of course, without this check, anything that
> overflows the bit field will trigger an EINVAL based on the use of
> FAN_INVALID_RESPONSE_MASK in process_access_response().
>
> Suggested-by: Steve Grubb <sgrubb@xxxxxxxxxx>
> Link: https://lore.kernel.org/r/2745105.e9J7NaK4W3@x2
> Suggested-by: Jan Kara <jack@xxxxxxx>
> Link: https://lore.kernel.org/r/20201001101219.GE17860@xxxxxxxxxxxxxx
> Signed-off-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
> Link: https://lore.kernel.org/r/17660b3f2817e5c0a19d1e9e5d40b53ff4561845.1651174324.git.rgb@xxxxxxxxxx
> ---
>  fs/notify/fanotify/fanotify.c      |  1 -
>  fs/notify/fanotify/fanotify.h      |  4 +-
>  fs/notify/fanotify/fanotify_user.c | 59 ++++++++++++++++++++----------
>  include/linux/fanotify.h           |  3 ++
>  include/uapi/linux/fanotify.h      | 27 +++++++++++++-
>  5 files changed, 72 insertions(+), 22 deletions(-)
>
> diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
> index 985e995d2a39..00aff6e29bf8 100644
> --- a/fs/notify/fanotify/fanotify.c
> +++ b/fs/notify/fanotify/fanotify.c
> @@ -266,7 +266,6 @@ static int fanotify_get_response(struct fsnotify_group *group,
>         case FAN_ALLOW:
>                 ret = 0;
>                 break;
> -       case FAN_DENY:

I personally would drop this from the patch if it was me, it doesn't
change the behavior so it falls under the "noise" category, which
could be a problem considering the lack of response on the original
posting and this one.  Small, focused patches have a better shot of
review/merging.

>         default:
>                 ret = -EPERM;
>         }

...

> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index 694516470660..f1ff4cf683fb 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -289,13 +289,19 @@ static int create_fd(struct fsnotify_group *group, struct path *path,
>   */
>  static void finish_permission_event(struct fsnotify_group *group,
>                                     struct fanotify_perm_event *event,
> -                                   __u32 response)
> +                                   struct fanotify_response *response)
>                                     __releases(&group->notification_lock)
>  {
>         bool destroy = false;
>
>         assert_spin_locked(&group->notification_lock);
> -       event->response = response;
> +       event->response = response->response;
> +       event->extra_info_type = response->extra_info_type;
> +       switch (event->extra_info_type) {
> +       case FAN_RESPONSE_INFO_AUDIT_RULE:
> +               memcpy(event->extra_info_buf, response->extra_info_buf,
> +                      sizeof(struct fanotify_response_audit_rule));

Since the fanotify_perm_event:extra_info_buf and
fanotify_response:extra_info_buf are the same type/length, and they
will be the same regardless of the extra_info_type field, why not
simply get rid of the above switch statement and do something like
this:

  memcpy(event->extra_info_buf, response->extra_info_buf,
         sizeof(response->extra_info_buf));

> +       }
>         if (event->state == FAN_EVENT_CANCELED)
>                 destroy = true;
>         else

...

> @@ -827,26 +845,25 @@ static ssize_t fanotify_read(struct file *file, char __user *buf,
>
>  static ssize_t fanotify_write(struct file *file, const char __user *buf, size_t count, loff_t *pos)
>  {
> -       struct fanotify_response response = { .fd = -1, .response = -1 };
> +       struct fanotify_response response;
>         struct fsnotify_group *group;
>         int ret;
> +       size_t size = min(count, sizeof(struct fanotify_response));
>
>         if (!IS_ENABLED(CONFIG_FANOTIFY_ACCESS_PERMISSIONS))
>                 return -EINVAL;
>
>         group = file->private_data;
>
> -       if (count < sizeof(response))
> +       if (count < offsetof(struct fanotify_response, extra_info_buf))
>                 return -EINVAL;

Is this why you decided to shrink the fanotify_response:response field
from 32-bits to 16-bits?  I hope not.  I would suggest both keeping
the existing response field as 32-bits and explicitly checking for
writes that are either the existing/compat length as well as the
newer, longer length.

> -       count = sizeof(response);
> -
>         pr_debug("%s: group=%p count=%zu\n", __func__, group, count);
>
> -       if (copy_from_user(&response, buf, count))
> +       if (copy_from_user(&response, buf, size))
>                 return -EFAULT;
>
> -       ret = process_access_response(group, &response);
> +       ret = process_access_response(group, &response, count);
>         if (ret < 0)
>                 count = ret;
>

...

> diff --git a/include/uapi/linux/fanotify.h b/include/uapi/linux/fanotify.h
> index e8ac38cc2fd6..efb5a3a6f814 100644
> --- a/include/uapi/linux/fanotify.h
> +++ b/include/uapi/linux/fanotify.h
> @@ -179,9 +179,34 @@ struct fanotify_event_info_error {
>         __u32 error_count;
>  };
>
> +/*
> + * User space may need to record additional information about its decision.
> + * The extra information type records what kind of information is included.
> + * The default is none. We also define an extra informaion buffer whose
> + * size is determined by the extra information type.
> + *
> + * If the context type is Rule, then the context following is the rule number
> + * that triggered the user space decision.
> + */
> +
> +#define FAN_RESPONSE_INFO_AUDIT_NONE   0
> +#define FAN_RESPONSE_INFO_AUDIT_RULE   1
> +
> +struct fanotify_response_audit_rule {
> +       __u32 rule;
> +};
> +
> +#define FANOTIFY_RESPONSE_EXTRA_LEN_MAX        \
> +       (sizeof(union { \
> +               struct fanotify_response_audit_rule r; \
> +               /* add other extra info structures here */ \
> +       }))
> +
>  struct fanotify_response {
>         __s32 fd;
> -       __u32 response;
> +       __u16 response;
> +       __u16 extra_info_type;
> +       char extra_info_buf[FANOTIFY_RESPONSE_EXTRA_LEN_MAX];
>  };

Since both the kernel and userspace are going to need to agree on the
content and formatting of the fanotify_response:extra_info_buf field,
why is it hidden behind a char array?  You might as well get rid of
that abstraction and put the union directly in the fanotify_response
struct.  It is possible you could also get rid of the
fanotify_response_audit_rule struct this way too and just access the
rule scalar directly.


--
paul-moore.com