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

[Nadav Amit]

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Sun, Jan 21, 2018 at 3:46 PM, Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
>> I wanted to see whether segments protection can be a replacement for PTI
>> (yes, excluding SMEP emulation), or whether speculative execution âignoresâ
>> limit checks, similarly to the way paging protection is skipped.
>> 
>> It does seem that segmentation provides sufficient protection from Meltdown.
>> The âreliabilityâ 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: wouldnât 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.

Thanks. Iâll try to check with Intel liaison people of VMware (my employer),
yet any feedback will be appreciated.

> Too bad x86-64 got rid of the segments ;)

Actually, as I noted in a different thread, running 32-bit binaries on
x86_64 in legacy-mode, without PTI, performs considerably better than x86_64
binaries with PTI for workloads that are hit the most (e.g., Redis). By
dynamically removing the 64-bit user-CS from the GDT, this mode should be
safe, as long as CS load is not done speculatively.

Regards,
Nadav