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

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Date:   Sat, 27 Apr 2019 06:59:36 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Eric Biggers <ebiggers@...nel.org>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        David Laight <David.Laight@...lab.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "luto@...nel.org" <luto@...nel.org>,
        "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>
Subject: Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall



> On Apr 26, 2019, at 11:02 AM, Theodore Ts'o <tytso@....edu> wrote:
> 
>> On Fri, Apr 26, 2019 at 10:44:20AM -0700, Eric Biggers wrote:
>> Would it be possibly to call ChaCha20 through the actual crypto API so that SIMD
>> instructions (e.g. AVX-2) could be used?  That would make it *much* faster.
>> Also consider AES-CTR with AES-NI instructions.
> 
> It's not obvious SIMD instructions will be faster in practice, since
> it requires saving and restoring the vector/FPU registers.  If you're
> going to be doing a *lot* of vector processing (for example when doing
> block-level RAID-5 / RAID-6 computations), it might be worth it.  But
> if you're only going to be turning the crank for 12 or 20 rounds, the
> overhead of calling kernel_fpu_begin() and kernel_fpu_end() is
> probably going to make this worth it.
> 

So generate a whole page or more of random bytes at a time and save them up for when they’re needed.