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SubjectRe: [PATCH 4/5] s390: define ISOLATE_BP to run tasks with modified branch prediction
On Tue, 23 Jan 2018 21:32:24 +0100
Radim Krčmář <> wrote:

> 2018-01-23 15:21+0100, Christian Borntraeger:
> > Paolo, Radim,
> >
> > this patch not only allows to isolate a userspace process, it also allows us
> > to add a new interface for KVM that would allow us to isolate a KVM guest CPU
> > to no longer being able to inject branches in any host or other guests. (while
> > at the same time QEMU and host kernel can run with full power).
> > We just have to set the TIF bit TIF_ISOLATE_BP_GUEST for the thread that runs a
> > given CPU. This would certainly be an addon patch on top of this patch at a later
> > point in time.
> I think that the default should be secure, so userspace will be
> breaking the isolation instead of setting it up and having just one
> place to screw up would be better -- the prctl could decide which
> isolation mode to pick.

The prctl is one direction only. Once a task is "secured" there is no way back.
If we start with a default of secure then *all* tasks will run with limited
branch prediction.

> Maybe we can change the conditions and break logical connection between
> TIF_ISOLATE_BP and TIF_ISOLATE_BP_GUEST, to make a separate KVM
> interface useful.

The thinking here is that you use TIF_ISOLATE_BP to make use space secure,
but you need to close the loophole that you can use a KVM guest to get out of
the secured mode. That is why you need to run the guest with isolated BP if
TIF_ISOLATE_BP is set. But if you want to run qemu as always and only the
KVM guest with isolataed BP you need a second bit, thus TIF_ISOLATE_GUEST_BP.

> > Do you think something similar would be useful for other architectures as well?
> It goes against my idea of virtualization, but there probably are users
> that don't care about isolation and still use virtual machines ...
> I expect most architectures to have a fairly similar resolution of
> branch prediction leaks, so the idea should be easily abstractable on
> all levels. (At least x86 is.)


> > In that case we should try to come up with a cross-architecture interface to enable
> > that.
> Makes me think of a generic VM control "prefer performance over
> security", which would also take care of future problems and let arches
> decide what is worth the code.

VM as in virtual machine or VM as in virtual memory?

> A main drawback is that this will introduce dynamic branches to the
> code, which are going to slow down the common case to speed up a niche.

Where would you place these additional branches? I don't quite get the idea.

blue skies,

"Reality continues to ruin my life." - Calvin.

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