linux-kernel - Re: [RFC PATCH] Implement /proc/pid/kill

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Message-ID: <20181031154932.GB21207@redhat.com>
Date:   Wed, 31 Oct 2018 16:49:33 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Christian Brauner <christian.brauner@...onical.com>
Subject: Re: [RFC PATCH] Implement /proc/pid/kill

On 10/31, Daniel Colascione wrote:
>
> > Confused... why? kill_ok_by_cred() should fail?
>
> Not if we don't run it. :-) I thought you were proposing that we do
> *all* access checks in open() and let write() succeed unconditionally,

Ah, no ;)

> Anyway, I sent a v2 patch that I think closes the hole another way. In
> v2, we just require that the real user ID that opens a /proc/pid/kill
> file is the same one that writes to it. It successfully blocks the
> setuid attack above while preserving all the write-time permission
> checks and keeping the close correspondence between
> write()-on-proc-pid-kill-fd and kill(2). Can you think of any
> situation where this scheme breaks?

I see no problems...

but again, perhaps we should fix kill_pid_info_as_cred() and use it in
/proc/pid/kill? I dunno.

Oleg.