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

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Date:	Fri, 06 Nov 2009 21:35:51 +0100
From:	matthieu castet <castet.matthieu@...e.fr>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Using x86 segments against NULL pointer deference exploit

Hi Alan,

Alan Cox wrote:
> On Fri, 06 Nov 2009 13:59:49 +0100
> castet.matthieu@...e.fr wrote:
> 
>> 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.
> 
> For one it is enormously expensive because the moment you have segment
> limits all sorts of stuff goes slower.
We can always imagine a lazy mechanism that will enable segment limit when page0 is mapped.
That will only slow down the machine when wine & co are running. 

> and for APM and so on.
APM clears all segment before calling bios (APM_ZERO_SEGS is defined for detecting buggy bios) 
and pnpbios seems to have their own segment (GDT_ENTRY_PNPBIOS) There is also GDT_ENTRY_APMBIOS_BASE,
but that seems unused.

> You also do sometimes need low 4K
> access for wine/dosemu etc as you guess - 
That's a bigger problem. If there not many access we can imagine fix it with trap/single step.

> 64bit you don't have a lot of those features ayway.
Yes.

May be the sane way should be to forbid mapping page 0, and make run application needing page 0 in a
emulator. After all it is for special case [1] :
- Win16 binary for wine
- upstream version of dosemu and qemu have workaround

But some distro still set mmap_min_addr to 0 (ubuntu+wine, ...) :(

Matthieu

[1] http://wiki.debian.org/mmap_min_addr
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