linux-kernel - Re: [PATCH v3 2/4] pid: Add PIDFD_IOCTL_GETFD to fetch file descriptors from processes

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Date:   Tue, 17 Dec 2019 02:13:16 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     kernel list <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Tycho Andersen <tycho@...ho.ws>,
        Aleksa Sarai <cyphar@...har.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        Al Viro <viro@...iv.linux.org.uk>, gpascutto@...illa.com,
        ealvarez@...illa.com, Florian Weimer <fweimer@...hat.com>,
        jld@...illa.com
Subject: Re: [PATCH v3 2/4] pid: Add PIDFD_IOCTL_GETFD to fetch file
 descriptors from processes

On Tue, Dec 17, 2019 at 2:00 AM Sargun Dhillon <sargun@...gun.me> wrote:
> This adds an ioctl which allows file descriptors to be extracted
> from processes based on their pidfd.
[...]
> You must have the ability to ptrace the process in order to extract any
> file descriptors from it. ptrace can already be used to extract file
> descriptors based on parasitic code injections, so the permissions
> model is aligned.
[...]
> +       task = get_pid_task(pid, PIDTYPE_PID);
> +       if (!task)
> +               return -ESRCH;
> +       ret = -EPERM;

Please add something like

if (mutex_lock_killable(&task->signal->cred_guard_mutex))
  goto out;

here, and drop the mutex after fget_task().

> +       if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
> +               goto out;
> +       ret = -EBADF;
> +       file = fget_task(task, args.fd);
> +       if (!file)
> +               goto out;
> +
> +       fd = get_unused_fd_flags(fd_flags);
> +       if (fd < 0) {
> +               ret = fd;
> +               goto out_put_file;
> +       }
> +       /*
> +        * security_file_receive must come last since it may have side effects
> +        * and cannot be reversed.
> +        */
> +       ret = security_file_receive(file);
> +       if (ret)
> +               goto out_put_fd;
> +
> +       fd_install(fd, file);
> +       put_task_struct(task);
> +       return fd;
> +
> +out_put_fd:
> +       put_unused_fd(fd);
> +out_put_file:
> +       fput(file);
> +out:
> +       put_task_struct(task);
> +       return ret;
> +}