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From: Andy Lutomirski <luto@kernel.org>
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>
Subject: Re: [PATCH v1 2/2] signal: add procfd_signal() syscall
Date: Mon, 19 Nov 2018 07:45:04 -0800
Message-ID: <CALCETrUY=Hk6=BjgPuDBgj5J1T_b5Q5u1uVOWHjGWXwmHoZBEQ@mail.gmail.com> (raw)
In-Reply-To: <20181119103241.5229-3-christian@brauner.io>

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?

  reply index

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-19 10:32 [PATCH v1 0/2] proc: allow signaling processes via file descriptors Christian Brauner
2018-11-19 10:32 ` [PATCH v1 1/2] proc: get process file descriptor from /proc/<pid> Christian Brauner
2018-11-19 15:32   ` Andy Lutomirski
2018-11-19 18:20     ` Christian Brauner
2018-11-19 10:32 ` [PATCH v1 2/2] signal: add procfd_signal() syscall Christian Brauner
2018-11-19 15:45   ` Andy Lutomirski [this message]
2018-11-19 15:57     ` Daniel Colascione
2018-11-19 18:39     ` Christian Brauner
2018-11-19 15:59   ` Daniel Colascione
2018-11-19 18:29     ` Christian Brauner
2018-11-19 19:02       ` Eric W. Biederman
2018-11-19 19:31         ` Christian Brauner
2018-11-19 19:39           ` Daniel Colascione
2018-11-19 17:10   ` Eugene Syromiatnikov
2018-11-19 18:23     ` Christian Brauner
2018-11-19 17:14   ` Eugene Syromiatnikov
2018-11-19 20:28   ` Aleksa Sarai
2018-11-19 20:55     ` Christian Brauner
2018-11-19 21:13       ` Christian Brauner
2018-11-19 21:18       ` Aleksa Sarai
2018-11-19 21:20         ` Christian Brauner
2018-11-19 21:21         ` Christian Brauner
2018-11-19 21:25           ` Aleksa Sarai
2018-11-19 21:26           ` Daniel Colascione
2018-11-19 21:36             ` Aleksa Sarai
2018-11-19 21:37             ` Christian Brauner
2018-11-19 21:41               ` Daniel Colascione
2018-11-20  4:59                 ` Eric W. Biederman
2018-11-20 10:31                   ` Christian Brauner
2018-11-21 21:39                     ` Serge E. Hallyn
2018-11-19 21:23         ` Aleksa Sarai
2018-11-22  7:41           ` Serge E. Hallyn
2018-11-19 22:39   ` Tycho Andersen
2018-11-19 22:49     ` Daniel Colascione
2018-11-19 23:07       ` Tycho Andersen
2018-11-20  0:27         ` Andy Lutomirski
2018-11-20  0:32           ` Christian Brauner
2018-11-20  0:34             ` Andy Lutomirski
2018-11-20  0:49           ` Daniel Colascione
2018-11-22  7:48     ` Serge E. Hallyn
2018-11-19 23:35   ` kbuild test robot
2018-11-19 23:37   ` kbuild test robot
2018-11-19 23:45     ` Christian Brauner
2018-11-28 21:45   ` Joey Pabalinas
2018-11-28 22:05     ` Christian Brauner
2018-11-28 23:02       ` Joey Pabalinas
2018-11-19 10:32 ` [PATCH] procfd_signal.2: document procfd_signal syscall Christian Brauner
2018-11-20 13:29   ` Michael Kerrisk (man-pages)
2018-11-28 20:59   ` Florian Weimer
2018-11-28 21:12     ` Christian Brauner

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