Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   David Laight <David.Laight@ACULAB.COM>
To:     'Ingo Molnar' <mingo@kernel.org>, Andy Lutomirski <luto@kernel.org>
CC:     "Reshetova, Elena" <elena.reshetova@intel.com>,
        Theodore Ts'o <tytso@mit.edu>,
        Eric Biggers <ebiggers3@gmail.com>,
        "ebiggers@google.com" <ebiggers@google.com>,
        "herbert@gondor.apana.org.au" <herbert@gondor.apana.org.au>,
        Peter Zijlstra <peterz@infradead.org>,
        "keescook@chromium.org" <keescook@chromium.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "jpoimboe@redhat.com" <jpoimboe@redhat.com>,
        "jannh@google.com" <jannh@google.com>,
        "Perla, Enrico" <enrico.perla@intel.com>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: RE: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall
Thread-Topic: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall
Thread-Index: AQHU9E1UquBTkhVACE2y3BuRFoekIqY8wW2AgAAdM1CAAXexAIAANZ3ggAAW1gCAAApRgIAAMeKAgAAd+PCAAQuGgIAAYQuAgAAKhwCACsPi4IADJTwAgAAcagCAAExngIAEBbGAgACIbACAAbyQ8IAA9yhggAF9YoCAABkBsIAAayebgAAR3KA=
Date:   Thu, 2 May 2019 16:34:46 +0000
Message-ID: <d64b3562d179430f9bdd8712999ff98a@AcuMS.aculab.com>
References: <2236FBA76BA1254E88B949DDB74E612BA4C63E24@IRSMSX102.ger.corp.intel.com>
 <20190426140102.GA4922@mit.edu>
 <57357E35-3D9B-4CA7-BAB9-0BE89E0094D2@amacapital.net>
 <2236FBA76BA1254E88B949DDB74E612BA4C66A8A@IRSMSX102.ger.corp.intel.com>
 <6860856C-6A92-4569-9CD8-FF6C5C441F30@amacapital.net>
 <2236FBA76BA1254E88B949DDB74E612BA4C6A4D7@IRSMSX102.ger.corp.intel.com>
 <303fc4ee5ac04e4fac104df1188952e8@AcuMS.aculab.com>
 <2236FBA76BA1254E88B949DDB74E612BA4C6C2C3@IRSMSX102.ger.corp.intel.com>
 <2e55aeb3b39440c0bebf47f0f9522dd8@AcuMS.aculab.com>
 <CALCETrXjGvWVgZHrKCfH6RBsnYOyD2+Mey1Esw7BsA4Eg6PS0A@mail.gmail.com>
 <20190502150853.GA16779@gmail.com>
In-Reply-To: <20190502150853.GA16779@gmail.com>
Accept-Language: en-GB, en-US
Content-Language: en-US
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8BIT
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: Ingo Molnar
> Sent: 02 May 2019 16:09
> * Andy Lutomirski <luto@kernel.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

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)