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

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Date:   Sun, 18 Nov 2018 21:43:19 +0100
From:   Christian Brauner <christian@...uner.io>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Daniel Colascione <dancol@...gle.com>,
        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 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());

> 
> > Yes, that's what I have in mind. A siginfo_t is small enough that we
> > could just store it as a blob allocated off the procfs inode or
> > something like that without bothering with a shmfs file. You'd be able
> > to read(2) the exit status as many times as you wanted.
> 
> I think that, if the syscall in question is read(2), then it should work *once* per struct file.  Otherwise running cat on the file would behave very oddly.
> 
> Read and poll have the same problem as write: we can’t check caps in read or poll either.