Re: Option to clear allocated kernel memory before freeing it?

[Chase Venters <chase.venters@clientec.com>]

On Mon, 19 Jun 2006, Michael Opdenacker wrote:

> Hello,
>
> Would it make sense to implement a kernel option that would clear kernel 
> memory before freeing it (by kfree or free_page(s))?
>
> Unless I'm missing something, uncleared memory previously used for kernel 
> allocations could later be recycled for user allocations, making it possible 
> for a user program to access sensitive driver data if it's lucky.

No sane operating system lets user-space have access to pages that haven't 
been cleared.

> Tough clearing memory should be efficient (thanks to the use of memset(), 
> optimized for each platform), there would of course be a significant 
> performance hit. However, this could be acceptable for systems with strong 
> security requirements...

A better way (and I believe the way Linux does it) is to map new pages to 
the zero page, read-only. If the process attempts to write to the zero 
page, we take a page fault and set up a new zeroed page, change the 
mapping, and return control back to that process.

> What do you think? If this idea makes sense, I'll be glad to help in 
> implementing it.
>
>   Thanks in advance,
>
>   Cheers,
>
>   Michael.

Thanks,
Chase
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