linux-kernel - Re: [PATCH v4 2/5] pid: Add PIDFD_IOCTL_GETFD to fetch file descriptors from processes

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Message-ID: <CALCETrUK-SHA=sOUrBscpf+Bpxxff2L3RpXEaAfRHNnHGxa-LQ@mail.gmail.com>
Date:   Thu, 19 Dec 2019 17:43:17 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux API <linux-api@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Tycho Andersen <tycho@...ho.ws>, Jann Horn <jannh@...gle.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>, gpascutto@...illa.com,
        ealvarez@...illa.com, Florian Weimer <fweimer@...hat.com>,
        jld@...illa.com, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v4 2/5] pid: Add PIDFD_IOCTL_GETFD to fetch file
 descriptors from processes

On Wed, Dec 18, 2019 at 3:55 PM Sargun Dhillon <sargun@...gun.me> wrote:
>
> +
> +       if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
> +               file = ERR_PTR(-EPERM);
> +               goto out;
> +       }

I don't think this is MODE_READ.  By copying an fd from the task, you
can easily change its state.

IMO it would be really nice if pidfd could act more like a capability
here and carry a ptrace mode, for example.  But I guess it doesn't
right now.


--Andy