linux-kernel - Re: [PATCH v3 00/11] mm: Hardened usercopy

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Date:	Wed, 20 Jul 2016 08:31:37 -0700
From:	Kees Cook <keescook@...omium.org>
To:	David Laight <David.Laight@...lab.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jan Kara <jack@...e.cz>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Will Deacon <will.deacon@....com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Russell King <linux@...linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Catalin Marinas <catalin.marinas@....com>,
	PaX Team <pageexec@...email.hu>, Borislav Petkov <bp@...e.de>,
	Mathias Krause <minipli@...glemail.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Tony Luck <tony.luck@...el.com>,
	Andy Lutomirski <luto@...nel.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Laura Abbott <labbott@...oraproject.org>,
	Brad Spengler <spender@...ecurity.net>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Pekka Enberg <penberg@...nel.org>,
	Daniel Micay <danielmicay@...il.com>,
	Casey Schaufler <casey@...aufler-ca.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v3 00/11] mm: Hardened usercopy

On Wed, Jul 20, 2016 at 2:52 AM, David Laight <David.Laight@...lab.com> wrote:
> From: Kees Cook
>> Sent: 15 July 2016 22:44
>> This is a start of the mainline port of PAX_USERCOPY[1].
> ...
>> - if address range is in the current process stack, it must be within the
>>   current stack frame (if such checking is possible) or at least entirely
>>   within the current process's stack.
> ...
>
> That description doesn't seem quite right to me.
> I presume the check is:
>   Within the current process's stack and not crossing the ends of the
>   current stack frame.

Actually, it's a bad description all around. :) The check is that the
range is within a valid stack frame (current or any prior caller's
frame). i.e. it does not cross a frame or touch the saved frame
pointer nor instruction pointer.

> The 'current' stack frame is likely to be that of copy_to/from_user().
> Even if you use the stack of the caller, any problematic buffers
> are likely to have been passed in from a calling function.
> So unless you are going to walk the stack (good luck on that)
> I'm not sure checking the stack frames is worth it.

Yup: that's exactly what it's doing: walking up the stack. :)

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security