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

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Date:   Mon, 6 May 2019 07:01:04 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     David Laight <David.Laight@...LAB.COM>,
        Ingo Molnar <mingo@...nel.org>
CC:     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

> From: Reshetova, Elena
> > Sent: 03 May 2019 17:17
> ...
> > rdrand (calling every 8 syscalls): Simple syscall: 0.0795 microseconds
> 
> You could try something like:
> 	u64 rand_val = cpu_var->syscall_rand
> 
> 	while (unlikely(rand_val == 0))
> 		rand_val = rdrand64();
> 
> 	stack_offset = rand_val & 0xff;
> 	rand_val >>= 6;
> 	if (likely(rand_val >= 4))
> 		cpu_var->syscall_rand = rand_val;
> 	else
> 		cpu_var->syscall_rand = rdrand64();
> 
> 	return stack_offset;
> 
> That gives you 10 system calls per rdrand instruction
> and mostly takes the latency out of line.

I am not really happy going the rdrand path for a couple of reasons:
- it is not available on older PCs
- its performance varies across CPUs that support it (and as I understood varies quite some)
- it is x86 centric and not generic

So, if we can use get_random_bytes() interface without tightening ourselves to
a particular instruction, I think it would be better. 
The numbers I have measured so far for buffer size of 4096 is SW only, 
I will try to measure today what boost (if any) we can have if we use SIMD 
code for it. 

Best Regards,
Elena.