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SubjectVirt Call depth tracking mitigation
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On 17/07/2022 00:17, Thomas Gleixner wrote:
> As IBRS is a performance horror show, Peter Zijstra and me revisited the
> call depth tracking approach and implemented it in a way which is hopefully
> more palatable and avoids the downsides of the original attempt.
>
> We both unsurprisingly hate the result with a passion...

And I hate to add more problems, but here we go.

Under virt, it's not just SMI's which might run behind your back. 
Regular interrupts/etc can probably be hand-waved away in the same way
that SMIs are.

Hypercalls however are a different matter.

Xen and HyperV both have hypercall pages, where the hypervisor provides
some executable code for the guest kernel to use.

Under the current scheme, the calls into the hypercall pages get
accounted, as objtool can see them, but the ret's don't.  This imbalance
is exasperated because some hypercalls are called in loops.

Worse however, it opens a hole where branch history is calculable and
the ret can reliably underflow.  This occurs when there's a minimal call
depth in Linux to get to the hypercall, and then a call depth of >16 in
the hypervisor.

The only variable in these cases is how much user control there is of
the registers, and I for one am not feeling lucky in face of the current
research.

The only solution I see here is for Linux to ret-thunk the hypercall
page too.  Under Xen, the hypercall page is mutable by the guest and
there is room to turn every ret into a jmp, but obviously none of this
is covered by any formal ABI, and this probably needs more careful
consideration than the short time I've put towards it.

That said, after a return from the hypervisor, Linux has no idea what
state the RSB is in, so the only safe course of action is to re-stuff.

CC'ing the HyperV folk for input on their side.

~Andrew
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