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

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Date:   Thu, 9 May 2019 07:59:15 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     David Laight <David.Laight@...LAB.COM>,
        Andy Lutomirski <luto@...nel.org>,
        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


* Reshetova, Elena <elena.reshetova@...el.com> wrote:

> > * Reshetova, Elena <elena.reshetova@...el.com> wrote:
> > 
> > > CONFIG_PAGE_TABLE_ISOLATION=n:
> > >
> > > base:                                  Simple syscall: 0.0510 microseconds
> > > get_random_bytes(4096 bytes buffer):   Simple syscall: 0.0597 microseconds
> > >
> > > So, pure speed wise get_random_bytes() with 1 page per-cpu buffer wins.
> > 
> > It still adds +17% overhead to the system call path, which is sad.
> > Why is it so expensive?
> 
> I guess I can experiment further with buffer size increase and/or 
> using HW acceleration (I mostly played around different rdrand paths now). 
> 
> What would be acceptable overheard approximately (so that I know how
> much I need to squeeze this thing)? 

As much as possible? No idea, I'm sad about anything that is more than 
0%, and I'd be *really* sad about anything more than say 1-2%.

I find it ridiculous that even with 4K blocked get_random_bytes(), which 
gives us 32k bits, which with 5 bits should amortize the RNG call to 
something like "once per 6553 calls", we still see 17% overhead? It's 
either a measurement artifact, or something doesn't compute.

Thanks,

	Ingo