linux-kernel - Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

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Date:   Tue, 30 Jul 2019 11:07:57 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>,
        Theodore Ts'o <tytso@....edu>,
        David Laight <David.Laight@...lab.com>,
        Eric Biggers <ebiggers3@...il.com>,
        "ebiggers@...gle.com" <ebiggers@...gle.com>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "Perla, Enrico" <enrico.perla@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

On Mon, Jul 29, 2019 at 11:41:11AM +0000, Reshetova, Elena wrote:
> I want to summarize here the data (including the performance numbers)
> and reasoning for the in-stack randomization feature. I have organized
> it in a simple set of Q&A below.

Thanks for these!

> The in-stack randomization is really a very small change both code wise and
> logic wise.
> It does not affect real workloads and does not require enablement of other
> features (such as GCC plugins).
> So, I think we should really reconsider its inclusion.

I'd agree: the code is tiny and while the benefit can't point to a
specific issue, it does point to the general weakness of the stack
offset being predictable which has been a core observation for many
stack-based attacks.

If we're going to save state between syscalls (like the 4096 random
bytes pool), how about instead we just use a single per-CPU long mixed
with rdtsc saved at syscall exit. That should be a reasonable balance
between all the considerations and make it trivial for the feature to
be a boot flag without the extra page of storage, etc.

-- 
Kees Cook