linux-kernel - Re: [PATCH v2 02/11] mm: Hardened usercopy

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Message-ID: <20160715125519.GA21685@350D>
Date:	Fri, 15 Jul 2016 22:55:19 +1000
From:	Balbir Singh <bsingharora@...il.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	Balbir Singh <bsingharora@...il.com>,
	Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Casey Schaufler <casey@...aufler-ca.com>,
	PaX Team <pageexec@...email.hu>,
	Brad Spengler <spender@...ecurity.net>,
	Russell King <linux@...linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Michael Ellerman <mpe@...erman.id.au>,
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	Fenghua Yu <fenghua.yu@...el.com>,
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	<linux-arm-kernel@...ts.infradead.org>, linux-ia64@...r.kernel.org,
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	<kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2 02/11] mm: Hardened usercopy

On Thu, Jul 14, 2016 at 09:53:31PM -0700, Kees Cook wrote:
> On Thu, Jul 14, 2016 at 9:05 PM, Kees Cook <keescook@...omium.org> wrote:
> > On Thu, Jul 14, 2016 at 6:41 PM, Balbir Singh <bsingharora@...il.com> wrote:
> >> 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?
> >
> > I'd be okay with expanding the test -- it should be an extremely rare
> > situation already since the common Reserved areas (kernel data) will
> > have already been explicitly tested.
> >
> > What's the best way to do "next page"? Should it just be:
> >
> > for ( ; page <= endpage ; ptr += PAGE_SIZE, page = virt_to_head_page(ptr) ) {
> >     if (!PageReserved(page))
> >         return "<spans multiple pages>";
> > }
> >
> > return NULL;
> >
> > ?
> 
> Er, I was testing the wrong thing. How about:
> 
>         /*
>          * Reject if range is not Reserved (i.e. special or device memory),
>          * since then the object spans several independently allocated pages.
>          */
>         for (; ptr <= end ; ptr += PAGE_SIZE, page = virt_to_head_page(ptr)) {
>                 if (!PageReserved(page))
>                         return "<spans multiple pages>";
>         }
> 
>         return NULL;

That looks reasonable to me

Balbir