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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 19 Nov 2018 07:45:04 -0800
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Subject: Re: [PATCH v1 2/2] signal: add procfd_signal() syscall
To:     Christian Brauner <christian@brauner.io>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Serge E. Hallyn" <serge@hallyn.com>, Jann Horn <jannh@google.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Daniel Colascione <dancol@google.com>,
        Tim Murray <timmurray@google.com>,
        linux-man <linux-man@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>
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On Mon, Nov 19, 2018 at 2:33 AM Christian Brauner <christian@brauner.io> wrote:
>
> The kill() syscall operates on process identifiers. After a process has
> exited its pid can be reused by another process. If a caller sends a signal
> to a reused pid it will end up signaling the wrong process. This issue has
> often surfaced and there has been a push [1] to address this problem.
>
> A prior patch has introduced the ability to get a file descriptor
> referencing struct pid by opening /proc/<pid>. This guarantees a stable
> handle on a process which can be used to send signals to the referenced
> process. Discussion has shown that a dedicated syscall is preferable over
> ioctl()s. Thus, the  new syscall procfd_signal() is introduced to solve
> this problem. It operates on a process file descriptor.
> The syscall takes an additional siginfo_t and flags argument. If siginfo_t
> is NULL then procfd_signal() behaves like kill() if it is not NULL it
> behaves like rt_sigqueueinfo.
> The flags argument is added to allow for future extensions of this syscall.
> It currently needs to be passed as 0.

A few questions.  First: you've made this work on /proc/PID, but
should it also work on /proc/PID/task/TID to send signals to a
specific thread?

> +bool proc_is_procfd(const struct file *file)
> +{
> +       return d_is_dir(file->f_path.dentry) &&
> +              (file->f_op == &proc_tgid_base_operations);
> +}

Maybe rename to proc_is_tgid_procfd() to leave room for proc_is_tid_procfd()?

> +       if (info) {
> +               ret = __copy_siginfo_from_user(sig, &kinfo, info);
> +               if (unlikely(ret))
> +                       goto err;
> +               /*
> +                * Not even root can pretend to send signals from the kernel.
> +                * Nor can they impersonate a kill()/tgkill(), which adds
> +                * source info.
> +                */
> +               ret = -EPERM;
> +               if ((kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL) &&
> +                   (task_pid(current) != pid))
> +                       goto err;

Is the exception for signaling yourself actually useful here?