linux-kernel - Re: Using x86 segments against NULL pointer deference exploit

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Message-ID: <87eiob76fh.fsf@basil.nowhere.org>
Date:	Fri, 06 Nov 2009 21:18:58 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	castet.matthieu@...e.fr
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Using x86 segments against NULL pointer deference exploit

castet.matthieu@...e.fr writes:

> Hi,
>
> I am wondering why we can't set the KERNEL_DS data segment to not contain the
> first page, ie changing it from R/W flat model to R/W expand down from
> 0xffffffff to 4096.

As Alan pointed out setting segment limits/bases has large penalties.

This has been already addressed by the mmap limit defaults 
on the VM level by disallowing to place something on the zero page.

In fact a lot of systems should already run with that default.

> PS : why x86_64 segment got access bit set and x86_32 doesn't ?

It's a extremly minor optimization, but the CPU sets it on the first
access anyways.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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