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

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Date:   Sun, 18 Nov 2018 17:16:23 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     Aleksa Sarai <cyphar@...har.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Christian Brauner <christian@...uner.io>,
        "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 4:53 PM, Daniel Colascione <dancol@...gle.com> wrote:
>> Sure, I'd propose that ptrace_may_access() is what we should use for
>> operation permission checks.
>
> The tricky part is that ptrace_may_access takes a struct task. We want
> logic that's *like* ptrace_may_access, but that works posthumously.
> It's especially tricky because there's an LSM hook that lets
> __ptrace_may_access do arbitrary things. And we can't just run that
> hook upon process death, since *after* a process dies, a process
> holding an exithand FD (or whatever we call it) may pass that FD to
> another process, and *that* process can read(2) from it.
>
> Another option is doing the exithand access check at open time. I
> think that's probably fine, and it would make things a lot simpler.
> But if we use this option, we should understand what we're doing, and
> get some security-conscious people to think through the implications.

A ptrace check is also probably too strict. Yama's ptrace_scope
feature will block ptrace between unrelated processes within a single
user context, but applying this restriction to exit code monitoring
seems too severe to me.