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

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Date:   Thu, 2 May 2019 16:34:46 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Ingo Molnar' <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>
CC:     "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

From: Ingo Molnar
> Sent: 02 May 2019 16:09
> * Andy Lutomirski <luto@...nel.org> wrote:
> 
> > Or we decide that calling get_random_bytes() is okay with IRQs off and
> > this all gets a bit simpler.
> 
> BTW., before we go down this path any further, is the plan to bind this
> feature to a real CPU-RNG capability, i.e. to the RDRAND instruction,
> which excludes a significant group of x86 of CPUs?

It has already been measured - it is far too slow.
Even just using 6 bits so it doesn't have to be read every system call is
probably a significant overhead (I don't think that was tested though).

I do agree that using 'real' randomness is probably OTT here.

> 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.

	David

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