Messages in this thread | | | Subject | Re: [RFC PATCH 3/3] nSVM: use svm->nested.save to load vmcb12 registers and avoid TOC/TOU races | From | Emanuele Giuseppe Esposito <> | Date | Tue, 14 Sep 2021 11:24:35 +0200 |
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On 14/09/2021 11:12, Maxim Levitsky wrote: > On Tue, 2021-09-14 at 12:02 +0300, Maxim Levitsky wrote: >> On Tue, 2021-09-14 at 10:20 +0200, Emanuele Giuseppe Esposito wrote: >>> On 12/09/2021 12:42, Maxim Levitsky wrote: >>>>> >>>>> - if (!nested_vmcb_valid_sregs(vcpu, &vmcb12->save) || >>>>> + if (!nested_vmcb_valid_sregs(vcpu, &svm->nested.save) || >>>>> !nested_vmcb_check_controls(vcpu, &svm->nested.ctl)) { >>>> If you use a different struct for the copied fields, then it makes >>>> sense IMHO to drop the 'control' parameter from nested_vmcb_check_controls, >>>> and just use the svm->nested.save there directly. >>>> >>> >>> Ok, what you say in patch 2 makes sense to me. I can create a new struct >>> vmcb_save_area_cached, but I need to keep nested.ctl because 1) it is >>> used also elsewhere, and different fields from the one checked here are >>> read/set and 2) using another structure (or the same >> >> Yes, keep nested.ctl, since vast majority of the fields are copied I think. > > But actually that you mention it, I'll say why not to create vmcb_control_area_cached > as well indeed and change the type of svm->nested.save to it. (in a separate patch) > > I see what you mean that we modify it a bit (but we shoudn't to be honest) and such, but > all of this can be fixed.
So basically you are proposing:
struct svm_nested_state { ... struct vmcb_control_area ctl; // we need this because it is used everywhere, I think struct vmcb_control_area_cached ctl_cached; struct vmcb_save_area_cached save_cached; ... }
and then
if (!nested_vmcb_valid_sregs(vcpu, &svm->nested.save_cached) || !nested_vmcb_check_controls(vcpu, &svm->nested.ctl_cached)) {
like that?
Or do you want to delete nested.ctl completely and just keep the fields actually used in ctl_cached?
Also, note that as I am trying to use vmcb_save_area_cached, it is worth noticing that nested_vmcb_valid_sregs() is also used in svm_set_nested_state(), so it requires some additional little changes.
Thank you, Emanuele
> > The advantage of having vmcb_control_area_cached is that it becomes impossible to use > by mistake a non copied field from the guest. > > It would also emphasize that this stuff came from the guest and should be treated as > a toxic waste. > > Note again that this should be done if we agree as a separate patch. > >> >> Best regards, >> Maxim Levitsky >> >> >>> vmcb_save_area_cached) in its place would just duplicate the same fields >>> of nested.ctl, creating even more confusion and possible inconsistency. >>> >>> Let me know if you disagree. >>> >>> Thank you, >>> Emanuele >>> > >
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