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

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Date:   Sun, 18 Nov 2018 12:54:10 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     Christian Brauner <christian@...uner.io>
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 12:43 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.

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

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?