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

[linux-os (Dick Johnson)]

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))?
>

No. Memory is cleared before being mapped to user-space. Memory
that is allocated for use by the kernel is never cleared by default.
To do so would waste valuable time for nothing gained.

> 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.

Wrong. You are missing a lot.

>
> 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...
>

Clearing, using the CPU is never efficient. That's why demand-zero
paging is used by many operating systems.

> What do you think? If this idea makes sense, I'll be glad to help in
> implementing it.
>
>    Thanks in advance,
>    Cheers,
>    Michael.
> --
> Michael Opdenacker, Free Electrons
> Free Embedded Linux Training Materials
> on http://free-electrons.com/training
> (More than 1000 pages!)

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.4 on an i686 machine (5592.72 BogoMips).
New book: http://www.AbominableFirebug.com/
_


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