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Date:   Wed, 31 Oct 2018 16:10:07 +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: <20181031151007.GA21207@redhat.com>
References: <20181029221037.87724-1-dancol@google.com>
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 <20181031124435.GA9007@redhat.com>
 <CAKOZuevTF2cjJ0-nK7nv7jbgFexyW-deScaYVzE3iEPC48SAJg@mail.gmail.com>
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On 10/31, Daniel Colascione wrote:
>
> > perhaps it would be simpler to do
> >
> >         my_cred = override_creds(file->f_cred);
> >         kill_pid(...);
> >         revert_creds(my_cred);
>
> Thanks for the suggestion. That looks neat, but it's not quite enough.
> The problem is that check_kill_permission looks for
> same_thread_group(current, t) _before_ checking kill_of_by_cred,

Yes, you are right.

Looks like kill_pid_info_as_cred() can find another user, but probably
it needs some changes with or without /proc/pid/kill ...

> There's another problem though: say we open /proc/pid/5/kill *, with
> proc 5 being an ordinary unprivileged process, e.g., the shell. At
> open(2) time, the access check passes. Now suppose PID 5 execve(2)s
> into a setuid process. The kill FD is still open, so the kill FD's
> holder can send a signal

Confused... why? kill_ok_by_cred() should fail?

Oleg.