# \[PATCH 7/9\] Pmalloc Rare Write: modify selected pools

- From: Igor Stoppa \<hidden\>
- Date: 2018-04-24 12:39:11 
- List: linux-security-module
- Page: https://ratatoskr.run/linux-security-module/2018/04/5249243
- Thread: https://ratatoskr.run/linux-security-module/2018/04/5249243/t.md

```text


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

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```