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Date:   Sun, 18 Nov 2018 21:43:19 +0100
From:   Christian Brauner <christian@brauner.io>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Daniel Colascione <dancol@google.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Andy Lutomirski <luto@kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Serge E. Hallyn" <serge@hallyn.com>, Jann Horn <jannh@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Tim Murray <timmurray@google.com>,
        Kees Cook <keescook@chromium.org>,
        Jan Engelhardt <jengelh@inai.de>
Subject: Re: [PATCH] proc: allow killing processes via file descriptors
Message-ID: <20181118204317.seaztq7fqmysucns@brauner.io>
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On Sun, Nov 18, 2018 at 01:28:41PM -0700, Andy Lutomirski wrote:
> 
> 
> > On Nov 18, 2018, at 12:44 PM, Daniel Colascione <dancol@google.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.