kernel-hardening - Re: [PATCH v8 3/8] security/brute: Detect a brute force attack

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Date: Sun, 4 Jul 2021 16:01:08 +0200
From: John Wood <john.wood@....com>
To: Alexander Lobakin <alobakin@...me>
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>,
	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>, Andi Kleen <ak@...ux.intel.com>,
	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 3/8] security/brute: Detect a brute force attack

On Sat, Jul 03, 2021 at 12:59:28PM +0200, John Wood wrote:
> Hi,
>
> On Fri, Jul 02, 2021 at 05:08:09PM +0000, Alexander Lobakin wrote:
> >
> > On the other hand, it leaves a potentional window for attackers to
> > perform brute force from xattr-incapable filesystems. So at the end
> > of the day I think that the current implementation (a strong
> > rejection of such filesystems) is way more secure than having
> > a fallback I proposed.
>
> I've been thinking more about this: that the Brute LSM depends on xattr
> support and I don't like this part. I want that brute force attacks can
> be detected and mitigated on every system (with minimal dependencies).
> So, now I am working in a solution without this drawback. I have some
> ideas but I need to work on it.

I have been coding and testing a bit my ideas but:

Trying to track the applications faults info using kernel memory ends up
in an easy to abuse system (denied of service due to large amount of memory
in use) :(

So, I continue with the v8 idea: xattr to track application crashes info.

> > I'm planning to make a patch which will eliminate such weird rootfs
> > type selection and just always use more feature-rich tmpfs if it's
> > compiled in. So, as an alternative, you could add it to your series
> > as a preparatory change and just add a Kconfig dependency on
> > CONFIG_TMPFS && CONFIG_TMPFS_XATTR to CONFIG_SECURITY_FORK_BRUTE
> > without messing with any fallbacks at all.
> > What do you think?
>
> Great. But I hope this patch will not be necessary for Brute LSM :)

My words are no longer valid ;)

Thanks,
John Wood

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