[PATCH] device_cgroup: Roll back to original exceptions after copy failure

[Paul Moore]

On Tue, Oct 25, 2022 at 7:02 AM Wang Weiyang <wangweiyang2 at huawei.com> wrote:
>
> When add the 'a *:* rwm' entry to devcgroup A's whitelist, at first A's
> exceptions will be cleaned and A's behavior is changed to
> DEVCG_DEFAULT_ALLOW. Then parent's exceptions will be copyed to A's
> whitelist. If copy failure occurs, just return leaving A to grant
> permissions to all devices. And A may grant more permissions than
> parent.
>
> Backup A's whitelist and recover original exceptions after copy
> failure.
>
> Fixes: 4cef7299b478 ("device_cgroup: add proper checking when changing default behavior")
> Signed-off-by: Wang Weiyang <wangweiyang2 at huawei.com>
> ---
>  security/device_cgroup.c | 33 +++++++++++++++++++++++++++++----
>  1 file changed, 29 insertions(+), 4 deletions(-)

On quick glance this looks reasonable to me, but I'm working with
limited time connected to a network so I can't say I've given this a
full and proper review; if a third party could spend some time to give
this an additional review before I merge it I would greatly appreciate
it.

> diff --git a/security/device_cgroup.c b/security/device_cgroup.c
> index a9f8c63a96d1..bef2b9285fb3 100644
> --- a/security/device_cgroup.c
> +++ b/security/device_cgroup.c
> @@ -82,6 +82,17 @@ static int dev_exceptions_copy(struct list_head *dest, struct list_head *orig)
>         return -ENOMEM;
>  }
>
> +static void dev_exceptions_move(struct list_head *dest, struct list_head *orig)
> +{
> +       struct dev_exception_item *ex, *tmp;
> +
> +       lockdep_assert_held(&devcgroup_mutex);
> +
> +       list_for_each_entry_safe(ex, tmp, orig, list) {
> +               list_move_tail(&ex->list, dest);
> +       }
> +}
> +
>  /*
>   * called under devcgroup_mutex
>   */
> @@ -604,11 +615,13 @@ static int devcgroup_update_access(struct dev_cgroup *devcgroup,
>         int count, rc = 0;
>         struct dev_exception_item ex;
>         struct dev_cgroup *parent = css_to_devcgroup(devcgroup->css.parent);
> +       struct dev_cgroup tmp_devcgrp;
>
>         if (!capable(CAP_SYS_ADMIN))
>                 return -EPERM;
>
>         memset(&ex, 0, sizeof(ex));
> +       memset(&tmp_devcgrp, 0, sizeof(tmp_devcgrp));
>         b = buffer;
>
>         switch (*b) {
> @@ -620,15 +633,27 @@ static int devcgroup_update_access(struct dev_cgroup *devcgroup,
>
>                         if (!may_allow_all(parent))
>                                 return -EPERM;
> -                       dev_exception_clean(devcgroup);
> -                       devcgroup->behavior = DEVCG_DEFAULT_ALLOW;
> -                       if (!parent)
> +                       if (!parent) {
> +                               devcgroup->behavior = DEVCG_DEFAULT_ALLOW;
> +                               dev_exception_clean(devcgroup);
>                                 break;
> +                       }
>
> +                       INIT_LIST_HEAD(&tmp_devcgrp.exceptions);
> +                       rc = dev_exceptions_copy(&tmp_devcgrp.exceptions,
> +                                                &devcgroup->exceptions);
> +                       if (rc)
> +                               return rc;
> +                       dev_exception_clean(devcgroup);
>                         rc = dev_exceptions_copy(&devcgroup->exceptions,
>                                                  &parent->exceptions);
> -                       if (rc)
> +                       if (rc) {
> +                               dev_exceptions_move(&devcgroup->exceptions,
> +                                                   &tmp_devcgrp.exceptions);
>                                 return rc;
> +                       }
> +                       devcgroup->behavior = DEVCG_DEFAULT_ALLOW;
> +                       dev_exception_clean(&tmp_devcgrp);
>                         break;
>                 case DEVCG_DENY:
>                         if (css_has_online_children(&devcgroup->css))
> --
> 2.17.1
>


-- 
paul-moore.com