linux-hardening - Re: [PATCH v7 0/7] Fork brute force attack mitigation

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Message-ID: <20210523154749.GA3762@ubuntu>
Date:   Sun, 23 May 2021 17:47:49 +0200
From:   John Wood <john.wood@....com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     John Wood <john.wood@....com>, Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Shuah Khan <shuah@...nel.org>, valdis.kletnieks@...edu,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-hardening@...r.kernel.org,
        kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v7 0/7] Fork brute force attack mitigation

On Sun, May 23, 2021 at 07:43:16AM -0700, Andi Kleen wrote:
>
> On 5/23/2021 12:31 AM, John Wood wrote:
> > Hi,
> >
> > On Fri, May 21, 2021 at 11:02:14AM -0700, Andi Kleen wrote:
> > > > Moreover, I think this solves another problem pointed out by Andi Kleen
> > > > during the v5 review [2] related to the possibility that a supervisor
> > > > respawns processes killed by the Brute LSM. He suggested adding some way so
> > > > a supervisor can know that a process has been killed by Brute and then
> > > > decide to respawn or not. So, now, the supervisor can read the brute xattr
> > > > of one executable and know if it is blocked by Brute and why (using the
> > > > statistical data).
> > > It looks better now, Thank.
> > >
> > > One potential problem is that the supervisor might see the executable
> > > directly, but run it through some wrapper. In fact I suspect that will be
> > > fairly common with complex daemons. So it couldn't directly look at the
> > > xattr. Might be useful to also pass this information through the wait*
> > > chain, so that the supervisor can directly collect it. That would need some
> > > extension to these system calls.
> > >
> > Could something like this help? (not tested)
>
> This works even when someone further down the chain died?

Yes, this is the idea. (but now is a work in progress :) )

> Assuming it does, for SIGCHLD it seems reasonable.

So, if there are no objections I will work on it for the next version.

>
> I'm not fully sure how it will interact with cgroup release tracking though,
> that might need more research (my understanding is that modern supervisors
> often use cgroups)

Yeah, a new topic to learn: cgroups. I will try to work on this too if there are
no objections.

Thanks for the feedback.
John Wood