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Date:   Wed, 9 Jun 2021 09:52:29 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     John Wood <john.wood@....com>, 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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Arnd Bergmann <arnd@...db.de>, valdis.kletnieks@...edu,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-hardening@...r.kernel.org,
        kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v8 0/8] Fork brute force attack mitigation

On Tue, Jun 08, 2021 at 04:38:15PM -0700, Andi Kleen wrote:
> 
> On 6/8/2021 4:19 PM, Kees Cook wrote:
> > On Sat, Jun 05, 2021 at 05:03:57PM +0200, John Wood wrote:
> > > [...]
> > > the kselftest to avoid the detection ;) ). So, in this version, to track
> > > all the statistical data (info related with application crashes), the
> > > extended attributes feature for the executable files are used. The xattr is
> > > also used to mark the executables as "not allowed" when an attack is
> > > detected. Then, the execve system call rely on this flag to avoid following
> > > executions of this file.
> > I have some concerns about this being actually usable and not creating
> > DoS situations. For example, let's say an attacker had found a hard-to-hit
> > bug in "sudo", and starts brute forcing it. When the brute LSM notices,
> > it'll make "sudo" unusable for the entire system, yes?
> > 
> > And a reboot won't fix it, either, IIUC.
> > 
> The whole point of the mitigation is to trade potential attacks against DOS.
> 
> If you're worried about DOS the whole thing is not for you.

Right, but there's no need to make a system unusable for everyone else.
There's nothing here that relaxes the defense (i.e. stop spawning apache
for 10 minutes). Writing it to disk with nothing that undoes it seems a
bit too much. :)

-- 
Kees Cook

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