Re: [patch 0/5] Support for sanitization flag in low-level pageallocator

From: Peter Zijlstra
Date: Sat May 30 2009 - 06:40:06 EST

On Fri, 2009-05-29 at 22:48 -0700, Larry H. wrote:
> On 07:32 Fri 29 May , Arjan van de Ven wrote:
> > On Thu, 28 May 2009 21:36:01 +0200
> > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > > > ... and if we zero on free, we don't need to zero on allocate.
> > > > While this is a little controversial, it does mean that at least
> > > > part of the cost is just time-shifted, which means it'll not be TOO
> > > > bad hopefully...
> > >
> > > zero on allocate has the advantage of cache hotness, we're going to
> > > use the memory, why else allocate it.
> Because zero on allocate kills the very purpose of this patch and it has
> obvious security implications. Like races (in information leak
> scenarios, that is). What happens in-between the release of the page and
> the new allocation that yields the same page? What happens if no further
> allocations happen in a while (that can return the old page again)?
> That's the idea.

I don't get it, these are in-kernel data leaks, you need to be able to
run kernel code to exploit these, if someone can run kernel code, you've
lost anyhow.

Why waste time on this?

> > So if you zero on free, the next allocation will reuse the zeroed page.
> > And due to LIFO that is not too far out "often", which makes it likely
> > the page is still in L2 cache.
> Thanks for pointing this out clearly, Arjan.

Thing is, the time between allocation and use is typically orders of
magnitude less than between free and use.

Really, get a life, go fix real bugs. Don't make our kernel slower for
wanking rights.

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