linux-kernel - Re: [PATCH 4.10 070/111] audit: fix auditd/kernel connection state tracking

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Message-ID: <CAHC9VhS40=qNxjNJSBqz2GwrqrvTqLKpdOWUrJ5iUDFp8ERO-A@mail.gmail.com>
Date:   Tue, 20 Feb 2018 09:51:08 -0500
From:   Paul Moore <paul@...l-moore.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Dmitry Vyukov <dvyukov@...gle.com>, linux-audit@...hat.com
Subject: Re: [PATCH 4.10 070/111] audit: fix auditd/kernel connection state tracking

On Tue, Feb 20, 2018 at 9:06 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Feb 20, 2018 at 08:25:21AM -0500, Paul Moore wrote:
>> On Tue, Feb 20, 2018 at 7:37 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Tue, Mar 28, 2017 at 02:30:56PM +0200, Greg Kroah-Hartman wrote:
>> >> 4.10-stable review patch.  If anyone has any objections, please let me know.
>> >
>> >> +     if (!(auditd_test_task(current) ||
>> >> +           (current == __mutex_owner(&audit_cmd_mutex)))) {
>> >> +             long stime = audit_backlog_wait_time;
>> >
>> > Since I cannot find the original email on lkml, NAK on this.
>> > __mutex_owner() is not a general purpose helper function.
>>
>> Since this code also exists in the current kernel, I need to ask what
>> recommended alternatives exist for determining the mutex owner?
>>
>> I imagine we could track the mutex owner separately in the audit
>> subsystem, but I'd much prefer to leverage an existing mechanism if
>> possible.
>
> It's not at all clear to me what that code does, I just stumbled upon
> __mutex_owner() outside of the mutex code itself and went WTF.

If you don't want people to use __mutex_owner() outside of the mutex
code I might suggest adding a rather serious comment at the top of the
function, because right now I don't see anything suggesting that
function shouldn't be used.  Yes, there is the double underscore
prefix, but that can mean a few different things these days.

> The comment (aside from having the most horribly style) ...

Yeah, your dog is ugly too.  Notice how neither comment is constructive?

> ... is wrong too, because it claims it will not block when we hold that lock, while,
> afaict, it will in fact do just that.

A mutex blocks when it is held, but the audit_log_start() function
should not block for the task that currently holds the
audit_cmd_mutex; that is what the comment is meant to convey.  I
believe the comment makes sense, but I did write it so I'll concede
that I'm probably the not best judge.  If anyone would like to offer a
different wording I'm happy to consider it.

> Maybe if you could explain how that code is supposed to work and why it
> doesn't know if it holds a lock I could make a suggestion...

I just spent a few minutes looking back over the bits available in
include/linux/mutex.h and I'm not seeing anything beyond
__mutex_owner() which would allow us to determine the mutex owning
task.  It's probably easiest for us to just track ownership ourselves.
I'll put together a patch later today.

-- 
paul moore
www.paul-moore.com