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Date: Fri, 15 Jul 2016 11:41:51 +1000
From: Balbir Singh <bsingharora@gmail.com>
To: Rik van Riel <riel@redhat.com>
Cc: bsingharora@gmail.com, Kees Cook <keescook@chromium.org>,
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Subject: Re: [PATCH v2 02/11] mm: Hardened usercopy
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On Thu, Jul 14, 2016 at 09:04:18PM -0400, Rik van Riel wrote:
> On Fri, 2016-07-15 at 09:20 +1000, Balbir Singh wrote:
> 
> > > ==
> > > +		   ((unsigned long)end & (unsigned
> > > long)PAGE_MASK)))
> > > +		return NULL;
> > > +
> > > +	/* Allow if start and end are inside the same compound
> > > page. */
> > > +	endpage = virt_to_head_page(end);
> > > +	if (likely(endpage == page))
> > > +		return NULL;
> > > +
> > > +	/* Allow special areas, device memory, and sometimes
> > > kernel data. */
> > > +	if (PageReserved(page) && PageReserved(endpage))
> > > +		return NULL;
> > 
> > If we came here, it's likely that endpage > page, do we need to check
> > that only the first and last pages are reserved? What about the ones
> > in
> > the middle?
> 
> I think this will be so rare, we can get away with just
> checking the beginning and the end.
>

But do we want to leave a hole where an aware user space
can try a longer copy_* to avoid this check? If it is unlikely
should we just bite the bullet and do the check for the entire
range?

Balbir Singh.