Messages in this thread | | | From | Paolo Bonzini <> | Subject | Re: [PATCH 2/2] KVM: nSVM: temporarly save vmcb12's efer, cr0 and cr4 to avoid TOC/TOU races | Date | Tue, 10 Aug 2021 11:15:55 +0200 |
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On 09/08/21 16:53, Emanuele Giuseppe Esposito wrote: > svm_switch_vmcb(svm, &svm->nested.vmcb02); > + > + /* Save vmcb12's EFER, CR0 and CR4 to avoid TOC/TOU races. */ > + vmcb12_efer = vmcb12->save.efer; > + vmcb12_cr0 = vmcb12->save.cr0; > + vmcb12_cr4 = vmcb12->save.cr4; > + > + if (!nested_vmcb_valid_sregs(vcpu, &vmcb12->save, vmcb12_efer, > + vmcb12_cr0, vmcb12_cr4) || > + !nested_vmcb_check_controls(vcpu, &svm->nested.ctl)) { > + vmcb12->control.exit_code = SVM_EXIT_ERR; > + vmcb12->control.exit_code_hi = 0; > + vmcb12->control.exit_info_1 = 0; > + vmcb12->control.exit_info_2 = 0; > + return 1; > + }
At this point you have already done a svm_switch_vmcb, so you need to undo its effects. This is indeed what returning 1 achieves. However, if you return 1 then the caller does:
if (enter_svm_guest_mode(vcpu, vmcb12_gpa, vmcb12)) { svm->nested.nested_run_pending = 0;
svm->vmcb->control.exit_code = SVM_EXIT_ERR; svm->vmcb->control.exit_code_hi = 0; svm->vmcb->control.exit_info_1 = 0; svm->vmcb->control.exit_info_2 = 0;
nested_svm_vmexit(svm); }
where we have three steps:
1) clearing nested_run_pending is all good
2) setting the exit code is good, but then you don't need to do it in enter_svm_guest_mode
3) nested_svm_vmexit is problematic; nested_svm_vmexit copies values from VMCB02 to VMCB12 but those have not been set yet (nested_vmcb02_prepare_save takes care of it). The simplest way to fix this is to add a bool argument to nested_svm_vmexit, saying whether the vmcb12 save area should be updated or not.
Paolo
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