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

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Date:   Thu, 9 May 2019 07:03:24 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     Ingo Molnar <mingo@...nel.org>
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


> > 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.
> 
> If you check what happens underneath of get_random_bytes(), there is
> a fair amount of stuff that is going on, including reseeding CRNG if reseeding
> interval has passed (see _extract_crng()). It also even attempts to stir in more
> entropy from rdrand if available:

Sorry pressed wrong button instead of copy pasting the code.
This is where it adds entropy:

if (arch_get_random_long(&v))
		crng->state[14] ^= v;


> I will look into this whole construction
> slowly now to investigate. I did't optimize anything yet also (I take 8 bits at
> the time for offset), but these small optimization won't make performance
> impact from 17% --> 2%, so pointless for now, need a more radical shift.
> 
> Best Regards,
> Elena.