linux-kernel - Re: [PATCH] proc: allow killing processes via file descriptors

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Date:   Sun, 18 Nov 2018 22:23:37 +0100
From:   Christian Brauner <christian@...uner.io>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Aleksa Sarai <cyphar@...har.com>,
        Andy Lutomirski <luto@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Jan Engelhardt <jengelh@...i.de>
Subject: Re: [PATCH] proc: allow killing processes via file descriptors

On Sun, Nov 18, 2018 at 12:54:10PM -0800, Daniel Colascione wrote:
> On Sun, Nov 18, 2018 at 12:43 PM, Christian Brauner
> <christian@...uner.io> wrote:
> > On Sun, Nov 18, 2018 at 01:28:41PM -0700, Andy Lutomirski wrote:
> >>
> >>
> >> > On Nov 18, 2018, at 12:44 PM, Daniel Colascione <dancol@...gle.com> wrote:
> >> >
> >>
> >> >
> >> > That is, I'm proposing an API that looks like this:
> >> >
> >> > int process_kill(int procfs_dfd, int signo, const union sigval value)
> >> >
> >> > If, later, process_kill were to *also* accept process-capability FDs,
> >> > nothing would break.
> >>
> >> Except that this makes it ambiguous to the caller as to whether their current creds are considered.  So it would need to be a different syscall or at least a flag.  Otherwise a lot of those nice theoretical properties go away.
> >
> > I can add a flag argument
> > int process_signal(int procfs_dfd, int signo, siginfo_t *info, int flags)
> > The way I see it process_signal() should be equivalent to kill(pid, signal) for now.
> > That is siginfo_t is cleared and set to:
> >
> > info.si_signo = sig;
> > info.si_errno = 0;
> > info.si_code = SI_USER;
> > info.si_pid = task_tgid_vnr(current);
> > info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
> 
> That makes sense. I just don't want to get into a situation where
> callers feel that they *have* to use the PID-based APIs to send a
> signal because process_kill doesn't offer some bit of functionality.

Yeah.

> 
> Are you imagining something like requiring info t be NULL unless flags
> contains some "I have a siginfo_t" value?

Well, I was actually thinking about something like:

/**
 *  sys_process_signal - send a signal to a process trough a process file descriptor
 *  @fd: the file descriptor of the process
 *  @sig: signal to be sent
 *  @info: the signal info
 *  @flags: future flags to be passed
 */
SYSCALL_DEFINE4(process_signal, int, fd, int, sig, siginfo_t __user *, info,
		int, flags)
{
	struct pid *pid;
	struct fd *f;
	kernel_siginfo_t kinfo;

	/* Do not allow users to pass garbage. */
	if (flags)
		return -EINVAL;

	int ret = __copy_siginfo_from_user(sig, &kinfo, info);
	if (unlikely(ret))
		return ret;

	/* For now, enforce that caller's creds are used. */
	kinfo.si_pid = task_tgid_vnr(current);
	kinfo.si_uid = from_kuid_munged(current_user_ns(), current_uid());

	if (signal_impersonates_kernel(kinfo))
		return -EPERM;

	f = fdget(fd);
	if (!f.file)
		return -EBADF;

	pid = f.file->private_data;
	if (!pid)
		return -EBADF;

	return kill_pid_info(sig, kinfo, pid);
}

> 
> BTW: passing SI_USER to rt_sigqueueinfo *should* as long as the
> passed-in si_pid and si_uid match what the kernel would set them to in
> the kill(2) case. The whole point of SI_USER is that the recipient
> knows that it can trust the origin information embedded in the
> siginfo_t in the signal handler. If the kernel verifies that a signal
> sender isn't actually lying, why not let people send SI_USER with
> rt_sigqueueinfo?