Re: [RFC PATCH 00/16] PTI support for x86-32

[hpa@zytor.com]

On January 21, 2018 6:11:07 PM PST, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>On Sun, Jan 21, 2018 at 3:46 PM, Nadav Amit <nadav.amit@gmail.com>
>wrote:
>> I wanted to see whether segments protection can be a replacement for
>PTI
>> (yes, excluding SMEP emulation), or whether speculative execution
>“ignores”
>> limit checks, similarly to the way paging protection is skipped.
>>
>> It does seem that segmentation provides sufficient protection from
>Meltdown.
>> The “reliability” test of Gratz PoC fails if the segment limit is set
>to
>> prevent access to the kernel memory. [ It passes if the limit is not
>set,
>> even if the DS is reloaded. ] My test is enclosed below.
>
>Interesting. It might not be entirely reliable for all
>microarchitectures, though.
>
>> So my question: wouldn’t it be much more efficient to use
>segmentation
>> protection for x86-32, and allow users to choose whether they want
>SMEP-like
>> protection if needed (and then enable PTI)?
>
>That's what we did long long ago, with user space segments actually
>using the limit (in fact, if you go back far enough, the kernel even
>used the base).
>
>You'd have to make sure that the LDT loading etc do not allow CPL3
>segments with base+limit past TASK_SIZE, so that people can't generate
>their own.  And the TLS segments also need to be limited (and
>remember, the limit has to be TASK_SIZE-base, not just TASK_SIZE).
>
>And we should check with Intel that segment limit checking really is
>guaranteed to be done before any access.
>
>Too bad x86-64 got rid of the segments ;)
>
>               Linus

No idea about Intel, but at least on Transmeta CPUs the limit check was asynchronous with the access.
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