linux-hardening - Re: [PATCH 4/4] usercopy: Remove HARDENED_USERCOPY

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Date:   Mon, 31 Jan 2022 19:27:35 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Kees Cook <keescook@...omium.org>, linux-mm@...ck.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 4/4] usercopy: Remove HARDENED_USERCOPY_PAGESPAN

On Mon, Jan 31, 2022 at 03:27:02PM +0100, David Hildenbrand wrote:
> On 11.01.22 00:15, Matthew Wilcox (Oracle) wrote:
> > There isn't enough information to make this a useful check any more;
> > the useful parts of it were moved in earlier patches, so remove this
> > set of checks now.
> > 
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> 
> 
> Very nice cleanup IMHO
> 
> Reviewed-by: David Hildenbrand <david@...hat.com>

Thanks.  I'm thinking about adding another check.  If the order-0
page has a non-NULL ->mapping, I believe that is enough to determine
that it's a page cache / KSM / Anon page, and as such is guaranteed
to actually be order-0 (and not part of a postive-order allocation
without __GFP_COMP set).  But that would require a bit more auditing,
or just throwing it in and seeing what breaks *whistles*.