[PATCH 2/4] KVM: PPC: Book3S HV: Fix radix guest SLB side channel

[Nicholas Piggin]

Excerpts from Paul Mackerras's message of February 10, 2021 11:28 am:
> On Mon, Jan 18, 2021 at 04:28:07PM +1000, Nicholas Piggin wrote:
>> The slbmte instruction is legal in radix mode, including radix guest
>> mode. This means radix guests can load the SLB with arbitrary data.
>> 
>> KVM host does not clear the SLB when exiting a guest if it was a
>> radix guest, which would allow a rogue radix guest to use the SLB as
>> a side channel to communicate with other guests.
> 
> No, because the code currently clears the SLB when entering a radix
> guest,

Not AFAIKS.

> which you remove in the next patch.

The next patch avoids clearing host SLB entries when a hash guest is 
entered from a radix host, it doesn't apply to radix guests. Not sure
where the changelog for it went but it should have "HPT guests" in the
title at least, I guess.

> I'm OK with moving the SLB
> clearing from guest entry to guest exit, I guess, but I don't see that
> you are in fact fixing anything by doing so.

I can set slb entries in a radix guest in simulator and observe they 
stay around over host<->guest transitions, and they don't after this
patch.

Thanks,
Nick