linux-kernel - Re: [kernel-hardening] Re: [PATCH v3 06/13] fork: Add generic vmalloced stack support

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Date:	Tue, 21 Jun 2016 12:43:56 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Rik van Riel <riel@...hat.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Jann Horn <jannh@...gle.com>,
	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Brian Gerst <brgerst@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jann Horn <jann@...jh.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [kernel-hardening] Re: [PATCH v3 06/13] fork: Add generic
 vmalloced stack support

On Tue, Jun 21, 2016 at 12:44 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Tuesday, June 21, 2016 2:32:28 PM CEST Rik van Riel wrote:
>> On Tue, 2016-06-21 at 10:13 -0700, Kees Cook wrote:
>> > On Tue, Jun 21, 2016 at 9:59 AM, Andy Lutomirski <luto@...capital.net
>> > > wrote:
>> > >
>> > > I'm tempted to explicitly disallow VM_NO_GUARD in the vmalloc
>> > > range.
>> > > It has no in-tree users for non-fixed addresses right now.
>> > What about the lack of pre-range guard page? That seems like a
>> > critical feature for this.
>>
>> If VM_NO_GUARD is disallowed, and every vmalloc area has
>> a guard area behind it, then every subsequent vmalloc area
>> will have a guard page ahead of it.
>>
>> I think disallowing VM_NO_GUARD will be all that is required.
>>
>> The only thing we may want to verify on the architectures that
>> we care about is that there is nothing mapped immediately before
>> the start of the vmalloc range, otherwise the first vmalloced
>> area will not have a guard page below it.
>
> FWIW, ARM has an 8MB guard area between the linear mapping of
> physical memory and the start of the vmalloc area. I have not
> checked any of the other architectures though.

If we start banning VM_NO_GUARD in the vmalloc area, we could also
explicitly prevent use of the bottom page of the vmalloc area.

>
>         Arnd



-- 
Andy Lutomirski
AMA Capital Management, LLC