linux-kernel - Re: [PATCH 2/5] seccomp: Introduce addfd ioctl to seccomp user notifier

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Message-ID: <CAMp4zn-F3b8Z4ZDjgnJM1Fbban2oVtCm_rZkhBhDKs6fw2_=rg@mail.gmail.com>
Date:   Sun, 24 May 2020 17:27:58 -0700
From:   Sargun Dhillon <sargun@...gun.me>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux API <linux-api@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Tycho Andersen <tycho@...ho.ws>,
        Kees Cook <keescook@...omium.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Jeffrey Vander Stoep <jeffv@...gle.com>,
        Jann Horn <jannh@...gle.com>, Robert Sesek <rsesek@...gle.com>,
        Chris Palmer <palmer@...gle.com>,
        Matt Denton <mpdenton@...gle.com>,
        Kees Cook <keescook@...gle.com>
Subject: Re: [PATCH 2/5] seccomp: Introduce addfd ioctl to seccomp user notifier

On Sun, May 24, 2020 at 5:05 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Sun, May 24, 2020 at 04:39:39PM -0700, Sargun Dhillon wrote:
>
> Bad refcounting rules.  *IF* we go with anything of that sort (and I'm not
> convinced that the entire series makes sense), it's better to have more
> uniform rules re reference consumption/disposal.
>
> Make the destructor of addfd *ALWAYS* drop its reference.  And have this
> function go
Are you suggesting the in both the error, and non-error cases the ioctl
invoker side is responsible for fput'ing the final reference in both the
success and non-success cases? Would we take an extra reference
prior to fd_install?
>
>         if (addfd->fd >= 0) {
>                 ret = replace_fd(addfd->fd, addfd->file, addfd->flags);
>         } else {
>                 ret = get_unused_fd_flags(addfd->flags);
>                 if (ret >= 0)
>                         fd_install(ret, get_file(addfd->file));
>         }
>
Wouldn't this result in consumption of reference in one case (fd_install),
and the fd still having a reference in the replace_fd case?