linux-kernel - Re: [PATCH 7/9] Pmalloc Rare Write: modify selected pools

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Date:   Tue, 24 Apr 2018 16:39:11 +0400
From:   Igor Stoppa <igor.stoppa@...il.com>
To:     lazytyped <lazytyped@...il.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     keescook@...omium.org, paul@...l-moore.com, sds@...ho.nsa.gov,
        mhocko@...nel.org, corbet@....net, labbott@...hat.com,
        david@...morbit.com, rppt@...ux.vnet.ibm.com,
        linux-security-module@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        Igor Stoppa <igor.stoppa@...wei.com>,
        Carlos Chinea Perez <carlos.chinea.perez@...wei.com>,
        Remi Denis Courmont <remi.denis.courmont@...wei.com>
Subject: Re: [PATCH 7/9] Pmalloc Rare Write: modify selected pools



On 24/04/18 16:32, lazytyped wrote:
> 
> 
> On 4/24/18 1:50 PM, Matthew Wilcox wrote:
>> struct modifiable_data {
>> 	struct immutable_data *d;
>> 	...
>> };
>>
>> Then allocate a new pool, change d and destroy the old pool.
> 
> With the above, you have just shifted the target of the arbitrary write
> from the immutable data itself to the pointer to the immutable data, so
> got no security benefit.
> 
> The goal of the patch is to reduce the window when stuff is writeable,
> so that an arbitrary write is likely to hit the time when data is read-only.

Indeed, that was my - poorly explained, I admit it - idea.

For example, that's the reason why I am remapping one page at a time in 
a loop, instead of doing the whole array, to limit exposure and increase 
randomness.

WRT the implementation, I'm sure there are bugs that need squashing.

But if I have overlooked some aspect in the overall design, I need 
guidance, because i still do not see what I am missing :-(

--
igor