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

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Message-ID: <CAGXu5jLuQPdBH4a0BF9AgH7qQubfoz+fFW2sTi2rRxsU8u_8QQ@mail.gmail.com>
Date:	Thu, 14 Jul 2016 21:53:31 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Balbir Singh <bsingharora@...il.com>
Cc:	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>,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	"x86@...nel.org" <x86@...nel.org>,
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	Pekka Enberg <penberg@...nel.org>,
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	Andrew Morton <akpm@...ux-foundation.org>,
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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 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;



-- 
Kees Cook
Chrome OS & Brillo Security