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

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Date:   Thu, 2 May 2019 18:43:25 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     David Laight <David.Laight@...lab.com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Theodore Ts'o <tytso@....edu>,
        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>,
        "keescook@...omium.org" <keescook@...omium.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


* Andy Lutomirski <luto@...nel.org> wrote:

> > 8 gigabits/sec sounds good throughput in principle, if there's no
> > scalability pathologies with that.
> 
> The latency is horrible.

Latency would be amortized via batching anyway, so 8 gigabits/sec 
suggests something on the order of magnitude of 4 bits per cycle, right? 
With 64 bits extraction at a time that would be 16 cycles per 64-bit 
word, which isn't too bad, is it?

But you are right that get_random_bytes() is probably faster, and also 
more generic.

> > It would also be nice to know whether RDRAND does buffering 
> > *internally*,
> 
> Not in a useful way :(

Too bad ...

> > Any non-CPU source of randomness for system calls and plans to add 
> > several extra function calls to every x86 system call is crazy talk I 
> > believe...
> 
> I think that, in practice, the only real downside to enabling this 
> thing will be the jitter in syscall times. Although we could decide 
> that the benefit is a bit dubious and the whole thing isn't worth it. 
> But it will definitely be optional.

Making it "optional" is not really a technical argument in any way 
though, either distros enable it in which case it's a de-facto default 
setting, or they don't, in which case it de-facto almost doesn't exist.

Thanks,

	Ingo