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Date: Sat, 30 May 2009 12:39:33 +0200 From: Peter Zijlstra <peterz@...radead.org> To: "Larry H." <research@...reption.com> Cc: Arjan van de Ven <arjan@...radead.org>, Alan Cox <alan@...rguk.ukuu.org.uk>, Ingo Molnar <mingo@...e.hu>, Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org, Linus Torvalds <torvalds@...l.org>, linux-mm@...ck.org, Ingo Molnar <mingo@...hat.com>, pageexec@...email.hu Subject: Re: [patch 0/5] Support for sanitization flag in low-level page allocator 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@...radead.org> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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