Re: [PATCH v4 0/6] Micro-optimize vclock_gettime

From: Thomas Gleixner
Date: Mon May 16 2011 - 17:53:19 EST

On Mon, 16 May 2011, Andi Kleen wrote:
> Andrew Lutomirski <luto@xxxxxxx> 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.


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