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SubjectRe: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs in mind (for KVM to isolate its guests per CPU)
On 10/15/2018 01:37 PM, Khalid Aziz wrote:
> On 09/24/2018 08:45 AM, Stecklina, Julian wrote:
>> I didn't test the version with TLB flushes, because it's clear that the
>> overhead is so bad that no one wants to use this.
> I don't think we can ignore the vulnerability caused by not flushing
> stale TLB entries. On a mostly idle system, TLB entries hang around long
> enough to make it fairly easy to exploit this. I was able to use the
> additional test in lkdtm module added by this patch series to
> successfully read pages unmapped from physmap by just waiting for system
> to become idle. A rogue program can simply monitor system load and mount
> its attack using ret2dir exploit when system is mostly idle. This brings
> us back to the prohibitive cost of TLB flushes. If we are unmapping a
> page from physmap every time the page is allocated to userspace, we are
> forced to incur the cost of TLB flushes in some way. Work Tycho was
> doing to implement Dave's suggestion can help here. Once Tycho has
> something working, I can measure overhead on my test machine. Tycho, I
> can help with your implementation if you need.

I looked at Tycho's last patch with batch update from
<>. I ported it on top of Julian's
patches and got it working well enough to gather performance numbers.
Here is what I see for system times on a machine with dual Xeon E5-2630
and 256GB of memory when running "make -j30 all" on 4.18.6 kernel
(percentages are relative to base 4.19-rc8 kernel without xpfo):

Base 4.19-rc8 913.84s
4.19-rc8 + xpfo, no TLB flush 1027.985s (+12.5%)
4.19-rc8 + batch update, no TLB flush 970.39s (+6.2%)
4.19-rc8 + xpfo, TLB flush 8458.449s (+825.6%)
4.19-rc8 + batch update, TLB flush 4665.659s (+410.6%)

Batch update is significant improvement but we are starting so far
behind baseline, it is still a huge slow down.


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