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Date: Mon, 16 May 2011 23:53:04 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Andi Kleen <andi@firstfloor.org>
cc: Andrew Lutomirski <luto@mit.edu>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
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        Eric Dumazet <eric.dumazet@gmail.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Borislav Petkov <bp@amd64.org>
Subject: Re: [PATCH v4 0/6] Micro-optimize vclock_gettime
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On Mon, 16 May 2011, Andi Kleen wrote:
> Andrew Lutomirski <luto@mit.edu> writes:
> > Longer term, it would be nice to mark the vsyscall page NX.  That
> > involves a few things:
> 
> Why NX? What would make sense is to call the VDSO from it.
> The problem is that the vDSO is randomized and there's no good memory
> location to store the pointer to it.
> 
> The real reason for all this dance is to have some less non randomized
> code around. What I implemented back then was instead code to patch out
> the SYSCALL in there if not needed to lower the attack surface (not sure
> if that still works though, but that was the idea). For most cases
> (TSC/HPET read) it's not needed.
> 
> Checking: someone removed the code meanwhile.

For a damned good reason.
 
> > And we won't have a
> > syscall instruction sitting at a predictable address.
> 
> The easy way to fix this is to just re-add the patching.

If you can address _all_ reasons why it was removed then we might
revisit that issue, but that's going to be an interesting patch.

Thanks,

	tglx