Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Wed, 31 Oct 2018 16:49:33 +0100
From:   Oleg Nesterov <oleg@redhat.com>
To:     Daniel Colascione <dancol@google.com>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Tim Murray <timmurray@google.com>,
        Joel Fernandes <joelaf@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Kees Cook <keescook@chromium.org>,
        Christian Brauner <christian.brauner@canonical.com>
Subject: Re: [RFC PATCH] Implement /proc/pid/kill
Message-ID: <20181031154932.GB21207@redhat.com>
References: <20181029221037.87724-1-dancol@google.com>
 <87bm7a3et9.fsf@xmission.com>
 <20181031124435.GA9007@redhat.com>
 <CAKOZuevTF2cjJ0-nK7nv7jbgFexyW-deScaYVzE3iEPC48SAJg@mail.gmail.com>
 <20181031151007.GA21207@redhat.com>
 <CAKOZues0_jMfW8xAa0mC=QS7UnHMzkWb5nCz3S_GDf3RzPg90Q@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAKOZues0_jMfW8xAa0mC=QS7UnHMzkWb5nCz3S_GDf3RzPg90Q@mail.gmail.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.