Re: Using x86 segments against NULL pointer deference exploit
From: Andi Kleen
Date: Fri Nov 06 2009 - 15:19:04 EST
> 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
ak@xxxxxxxxxxxxxxx -- Speaking for myself only.
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