cxl: fix setting of _PAGE_USER bit when handling page faults

[Michael Ellerman]

On Mon, 2016-04-11 at 10:01 +0530, Aneesh Kumar K.V wrote:
> Michael Ellerman <mpe at ellerman.id.au> writes:
> > On Mon, 2016-04-11 at 14:10 +1000, Andrew Donnellan wrote:
> > > On 29/03/16 00:42, Aneesh Kumar K.V wrote:
> > > > I noticed this when doing radix support and have a variant posted at
> > > > 
> > > > https://lists.ozlabs.org/pipermail/linuxppc-dev/2016-March/141036.html
> > > 
> > > I'm happy for this to be fixed in your radix series.
> > 
> > I'm not :)
> > 
> > This needs a stand-alone fix that we can backport.
> 
> It is done as an independent patch 
> 
> http://mid.gmane.org/1460182444-2468-2-git-send-email-aneesh.kumar@linux.vnet.ibm.com

Sure, but that's still not a *fix*.

A fix is a single commit, preferably with a subject that literally contains the
word "fix" or "bug", which fixes just the bug and nothing else. It should also
have a Fixes: line, if possible, and a Cc stable if appropriate.

It should also describe clearly what the bug is, why it's serious or just
annoying or whatever.

In this case it *looks* like we have a giant hole in the mm handling for CAPI
contexts, which would let userspace create mappings of kernel memory with
_PAGE_USER set. I think I agree with Ian that in fact that's not true, but it's
not clear from the diff that is the case. So I'd really like someone to write a
good commit message demonstrating that we understand what the bug is and why
it's not a big deal, despite the patch looking scary at first glance.

</grumpy> :)

cheers