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

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Message-ID: <CALCETrVBXZNAKGRXm0_txKGjqKnjx30Eb05hesye8M50D4A8Mw@mail.gmail.com>
Date:   Thu, 2 May 2019 09:32:35 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        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

On Thu, May 2, 2019 at 8:09 AM Ingo Molnar <mingo@...nel.org> wrote:
>
>
> * 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's kind of the opposite.  Elena benchmarked it, and RDRAND's
performance was truly awful 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 ...

There's no such thing as "starving" other users in this context.  The
current core RNG code uses a cryptographic RNG that has no limits on
the number of bytes extracted.  If you want the entropy-accounted
stuff, you can use /dev/random, which is separate.

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

The latency is horrible.

>
> It would also be nice to know whether RDRAND does buffering *internally*,

Not in a useful way :(

> 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.