On Mon, Nov 11, 2019 at 05:01:58PM -0800, Ram Pai wrote: > On Mon, Nov 11, 2019 at 03:19:24PM +1100, Paul Mackerras wrote: > > On Mon, Nov 04, 2019 at 09:47:59AM +0530, Bharata B Rao wrote: > > > From: Sukadev Bhattiprolu <sukadev@xxxxxxxxxxxxx> > > > > > > Implement the H_SVM_INIT_ABORT hcall which the Ultravisor can use to > > > abort an SVM after it has issued the H_SVM_INIT_START and before the > > > H_SVM_INIT_DONE hcalls. This hcall could be used when Ultravisor > > > encounters security violations or other errors when starting an SVM. > > > > > > Note that this hcall is different from UV_SVM_TERMINATE ucall which > > > is used by HV to terminate/cleanup an SVM. > > > > > > In case of H_SVM_INIT_ABORT, we should page-out all the pages back to > > > HV (i.e., we should not skip the page-out). Otherwise the VM's pages, > > > possibly including its text/data would be stuck in secure memory. > > > Since the SVM did not go secure, its MSR_S bit will be clear and the > > > VM wont be able to access its pages even to do a clean exit. > > > > It seems fragile to me to have one more transfer back into the > > ultravisor after this call. Why does the UV need to do this call and > > then get control back again one more time? > > Why can't the UV defer > > doing this call until it can do it without expecting to see a return > > from the hcall? > > Sure. But, what if the hypervisor calls back into the UV through a > ucall, asking for some page to be paged-out? If the ultravisor has > cleaned up the state associated with the SVM, it wont be able to service > that request. > > H_SVM_INIT_ABORT is invoked to tell the hypervisor that the > secure-state-transition for the VM cannot be continued any further. > Hypervisor can than choose to do whatever with that information. It can > cleanup its state, and/or make ucalls to get some information from the > ultravisor. It can also choose not to return control back to the ultravisor. > > > > And if it does need to see a return from the hcall, > > what would happen if a malicious hypervisor doesn't do the return? > > That is fine. At most it will be a denail-of-service attack. > > RP > > > > > Paul. > > > > > > If the ultravisor cleans up the SVM's state on its side and then informs > the Hypervisor to abort the SVM, the hypervisor will not be able to > cleanly terminate the VM. Because to terminate the SVM, the hypervisor > still needs the services of the Ultravisor. For example: to get the > pages back into the hypervisor if needed. Another example is, the > hypervisor can call UV_SVM_TERMINATE. Regardless of which ucall > gets called, the ultravisor has to hold on to enough state of the > SVM to service that request. OK, that's a good reason. That should be explained in the commit message. > The current design assumes that the hypervisor explicitly informs the > ultravisor, that it is done with the SVM, through the UV_SVM_TERMINATE > ucall. Till that point the Ultravisor must to be ready to service any > ucalls made by the hypervisor on the SVM's behalf. I see that UV_SVM_TERMINATE is done when the VM is being destroyed (at which point kvm->arch.secure_guest doesn't matter any more), and in kvmhv_svm_off(), where kvm->arch.secure_guest gets cleared explicitly. Hence I don't see any need for clearing it in the assembly code on the next secure guest entry. I think the change to book3s_hv_rmhandlers.S can just be dropped. Paul.
