Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: Virt Call depth tracking mitigation | Date | Tue, 19 Jul 2022 16:13:16 +0200 |
| |
On Tue, Jul 19 2022 at 10:24, Andrew Cooper wrote: > 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.
You mean host side interrupts, right?
> 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.
Bah.
> 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.
Well, that makes the guest side "safe", but isn't a deep hypercall > 16 already underflowing in the hypervisor code before it returns to the guest?
> 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.
Indeed.
Another proof for my claim that virt creates more problems than it solves.
Thanks,
tglx
| |