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 <20181031151007.GA21207@redhat.com>
From:   Daniel Colascione <dancol@google.com>
Date:   Wed, 31 Oct 2018 15:16:32 +0000
Message-ID: <CAKOZues0_jMfW8xAa0mC=QS7UnHMzkWb5nCz3S_GDf3RzPg90Q@mail.gmail.com>
Subject: Re: [RFC PATCH] Implement /proc/pid/kill
To:     Oleg Nesterov <oleg@redhat.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>
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On Wed, Oct 31, 2018 at 3:10 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> 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?

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,
since that's the model that a lot of FD-mediated resources (like
regular files) use. (MAC notwithstanding.)

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 *think* comparing struct user
addresses instead of numeric UIDs will protect the check against user
namespace shenanigans.