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

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Message-ID: <20190502164524.GB115950@gmail.com>
Date:   Thu, 2 May 2019 18:45:24 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "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


* David Laight <David.Laight@...LAB.COM> wrote:

> It has already been measured - it is far too slow.

I don't think proper buffering was tested, was it? Only a per syscall 
RDRAND overhead which I can imagine being not too good.

> > Because calling tens of millions of system calls per second will 
> > deplete any non-CPU-RNG sources of entropy and will also starve all 
> > other users of random numbers, which might have a more legitimate 
> > need for randomness, such as the networking stack ...
> 
> If the function you use to generate random numbers from the 'entropy 
> pool' isn't reversible (in a finite time) I don't think you really need 
> to worry about bits-in v bits-out.

Ok.

Thanks,

	Ingo