linux-kernel - Re: [PATCH v7 5/6] seccomp: add a way to pass FDs via a notification fd

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Message-ID: <CAG48ez0JY=_fvgy+t92-08kBFgTAZ-3hgAp+OOAwmSd73MJ-QA@mail.gmail.com>
Date:   Thu, 27 Sep 2018 21:28:07 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Tycho Andersen <tycho@...ho.ws>
Cc:     Kees Cook <keescook@...omium.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        containers@...ts.linux-foundation.org,
        Linux API <linux-api@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Tyler Hicks <tyhicks@...onical.com>,
        suda.akihiro@....ntt.co.jp, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v7 5/6] seccomp: add a way to pass FDs via a notification fd

On Thu, Sep 27, 2018 at 5:11 PM Tycho Andersen <tycho@...ho.ws> wrote:
> This patch adds a way to insert FDs into the tracee's process (also
> close/overwrite fds for the tracee). This functionality is necessary to
> mock things like socketpair() or dup2() or similar, but since it depends on
> external (vfs) patches, I've left it as a separate patch as before so the
> core functionality can still be merged while we argue about this. Except
> this time it doesn't add any ugliness to the API :)
[...]
> +static long seccomp_notify_put_fd(struct seccomp_filter *filter,
> +                                 unsigned long arg)
> +{
> +       struct seccomp_notif_put_fd req;
> +       void __user *buf = (void __user *)arg;
> +       struct seccomp_knotif *knotif = NULL;
> +       long ret;
> +
> +       if (copy_from_user(&req, buf, sizeof(req)))
> +               return -EFAULT;
> +
> +       if (req.fd < 0 && req.to_replace < 0)
> +               return -EINVAL;
> +
> +       ret = mutex_lock_interruptible(&filter->notify_lock);
> +       if (ret < 0)
> +               return ret;
> +
> +       ret = -ENOENT;
> +       list_for_each_entry(knotif, &filter->notif->notifications, list) {
> +               struct file *file = NULL;
> +
> +               if (knotif->id != req.id)
> +                       continue;

Are you intentionally permitting non-SENT states here? It shouldn't
make a big difference, but I think it'd be nice to at least block the
use of notifications in SECCOMP_NOTIFY_REPLIED state.

> +               if (req.fd >= 0)
> +                       file = fget(req.fd);