Prevent list poison values from being mapped by userspace processes

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Mon, Aug 24, 2015 at 12:22:33PM -0700, Kees Cook wrote:
> On Mon, Aug 24, 2015 at 12:14 PM, Russell King - ARM Linux
> <linux@arm.linux.org.uk> wrote:
> > On Mon, Aug 24, 2015 at 11:51:04AM -0700, Kees Cook wrote:
> >> On Mon, Aug 24, 2015 at 11:47 AM, Russell King - ARM Linux
> >> <linux@arm.linux.org.uk> wrote:
> >> > That's something which Catalin indicated that he'll work on.  However,
> >> > he said in a public email last week that he won't be around for a while.
> >> >
> >> > So, I have no immediate solution for LPAE - it looks like LPAE will
> >> > require switching the page tables on kernel entry or exit, and again
> >> > each and every time we want to perform a userspace access.  How this
> >> > is done is not something that has been discussed, and neither do we
> >> > yet know how expensive this will be.  There are a number of places in
> >> > the kernel where a large number of get_user()s or put_user()s follow
> >> > one after each other, which necessitates switching back and forth
> >> > multiple times.  We may need to address some of those areas by
> >> > converting them to copy_(to|from)_user().
> >>
> >> By the way, have you looked at grsecurity's implementation of these
> >> protections? They've been using domains for a while now, and I think
> >> have an LPAE solution as well.
> >
> > *Sigh*.
> >
> > No, and I really don't care - if people want to do development work out
> > of the mainline kernel and not bother to talk about getting it upstream,
> > it's their loss.  As far as I'm concerned, such external work doesn't
> > exist.
> 
> Sure, I understand, but it's worth at least looking at to compare
> feature sets. For example, when doing the W^X kernel memory work, I
> looked at both qcom and spender's work, trying to get the best of both
> into upstreamable shape.

That's one way of looking at it.

Another way of looking at it is that by looking at their work, and
merging their ideas into your own, it becomes an encouragement for
working outside of mainline - not only do they get the kernel itself
free, but they get their feature merged without themselves doing any
work - while some other bugger has to sort out making their code
mergable.

Therefore, my standard point of view is that if people can't be
bothered to talk about their ARM specific kernel features here with
a view to having them merged, they are leeching off the efforts of
the upstream kernel community, and their code just isn't worth
looking at.

I hold the same view on "community" kernel trees which don't bother
pushing their code upstream as well.

Sorry, I'm *not* supporting leeches.

I've already been accused this year by one very mistaken individual
for not pushing _my_ iMX6 work into community kernel trees - when
the work that I do is solely targetted at mainline kernels.  The
leeches are going mad, and I'm saying no more to this crap.  If it's
not talked about on a recognised mainline kernel mailing list, it
doesn't exist, and deserves to be rewritten.

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