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

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Date:   Wed, 17 Apr 2019 08:24:54 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Theodore Ts'o <tytso@....edu>,
        David Laight <David.Laight@...LAB.COM>,
        'Peter Zijlstra' <peterz@...radead.org>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        "luto@...nel.org" <luto@...nel.org>,
        "luto@...capital.net" <luto@...capital.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "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>
Subject: Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

* Theodore Ts'o <tytso@....edu> wrote:

> It seems though the assumption that we're assuming the attacker has 
> arbitrary ability to get the low bits of the stack, so *if* that's 
> true, then eventually, you'd be able to get enough samples that you 
> could reverse engineer the prandom state.  This could take long enough 
> that the process will have gotten rescheduled to another CPU, and since 
> the prandom state is per-cpu, that adds another wrinkle.

Yeah.

Note that if the attacker has this level of local access then they can 
probably also bind the task to a CPU, which would increase the 
statistical stability of any attack. Plus with millions of system calls 
per second executed in an attack, each of which system call exposes a 
couple of bits of prandom state, I'm pretty sure some prandom attack 
exists that can make the extraction of the full internal state probable 
within the ~60 seconds reseeding interval. (Is there any research on this 
perhaps, or do researchers not even bother, because this isn't really a 
secure algorithm in any reasonable meaning of the word?)

Thanks,

	Ingo