Messages in this thread | | | Date | Mon, 4 Apr 2022 22:44:04 +0200 | From | "Maciej S. Szmigiero" <> | Subject | Re: [PATCH 5/8] KVM: SVM: Re-inject INT3/INTO instead of retrying the instruction |
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On 4.04.2022 18:49, Sean Christopherson wrote: > On Mon, Apr 04, 2022, Maxim Levitsky wrote: >> On Sat, 2022-04-02 at 01:09 +0000, Sean Christopherson wrote: >>> Re-inject INT3/INTO instead of retrying the instruction if the CPU >>> encountered an intercepted exception while vectoring the software >>> exception, e.g. if vectoring INT3 encounters a #PF and KVM is using >>> shadow paging. Retrying the instruction is architecturally wrong, e.g. >>> will result in a spurious #DB if there's a code breakpoint on the INT3/O, >>> and lack of re-injection also breaks nested virtualization, e.g. if L1 >>> injects a software exception and vectoring the injected exception >>> encounters an exception that is intercepted by L0 but not L1. >>> >>> Due to, ahem, deficiencies in the SVM architecture, acquiring the next >>> RIP may require flowing through the emulator even if NRIPS is supported, >>> as the CPU clears next_rip if the VM-Exit is due to an exception other >>> than "exceptions caused by the INT3, INTO, and BOUND instructions". To >>> deal with this, "skip" the instruction to calculate next_ript, and then >>> unwind the RIP write and any side effects (RFLAGS updates). > > ... > (..) >>> + >>> kvm_make_request(KVM_REQ_EVENT, vcpu); >>> >>> vector = exitintinfo & SVM_EXITINTINFO_VEC_MASK; >>> @@ -3711,9 +3762,9 @@ static void svm_complete_interrupts(struct kvm_vcpu *vcpu) >>> * hit a #NP in the guest, and the #NP encountered a #PF, the #NP will >>> * be the reported vectored event, but RIP still needs to be unwound. >>> */ >>> - if (int3_injected && type == SVM_EXITINTINFO_TYPE_EXEPT && >>> - kvm_is_linear_rip(vcpu, svm->int3_rip)) >>> - kvm_rip_write(vcpu, kvm_rip_read(vcpu) - int3_injected); >>> + if (soft_int_injected && type == SVM_EXITINTINFO_TYPE_EXEPT && >>> + kvm_is_linear_rip(vcpu, svm->soft_int_linear_rip)) >>> + kvm_rip_write(vcpu, kvm_rip_read(vcpu) - soft_int_injected); >>> >>> switch (type) { >>> case SVM_EXITINTINFO_TYPE_NMI: >>> @@ -3726,14 +3777,6 @@ static void svm_complete_interrupts(struct kvm_vcpu *vcpu) >>> if (vector == X86_TRAP_VC) >>> break; >>> >>> - /* >>> - * In case of software exceptions, do not reinject the vector, >>> - * but re-execute the instruction instead. Rewind RIP first >>> - * if we emulated INT3 before. >>> - */ >>> - if (kvm_exception_is_soft(vector)) >>> - break; >>> - >>> if (exitintinfo & SVM_EXITINTINFO_VALID_ERR) { >>> u32 err = svm->vmcb->control.exit_int_info_err; >>> kvm_requeue_exception_e(vcpu, vector, err); >>> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h >>> index 47e7427d0395..a770a1c7ddd2 100644 >>> --- a/arch/x86/kvm/svm/svm.h >>> +++ b/arch/x86/kvm/svm/svm.h >>> @@ -230,8 +230,8 @@ struct vcpu_svm { >>> bool nmi_singlestep; >>> u64 nmi_singlestep_guest_rflags; >>> >>> - unsigned int3_injected; >>> - unsigned long int3_rip; >>> + unsigned soft_int_injected; >>> + unsigned long soft_int_linear_rip; >>> >>> /* optional nested SVM features that are enabled for this guest */ >>> bool nrips_enabled : 1; >> >> >> I mostly agree with this patch, but think that it doesn't address the >> original issue that Maciej wanted to address: >> >> Suppose that there is *no* instruction in L2 code which caused the software >> exception, but rather L1 set arbitrary next_rip, and set EVENTINJ to software >> exception with some vector, and that injection got interrupted. >> >> I don't think that this code will support this. > > Argh, you're right. Maciej's selftest injects without an instruction, but it doesn't > configure the scenario where that injection fails due to an exception+VM-Exit that > isn't intercepted by L1 and is handled by L0. The event_inj test gets the coverage > for the latter, but always has a backing instruction. > >> I think that svm_complete_interrupts should store next_rip it in some field >> like VMX does (vcpu->arch.event_exit_inst_len). > > Yeah. The ugly part is that because next_rip is guaranteed to be cleared on exit > (the exit is gauranteed to be due to a fault-like exception), KVM has to snapshot > next_rip during the "original" injection and use the linear_rip matching heuristic > to detect this scenario. > >> That field also should be migrated, or we must prove that it works anyway. >> E.g, what happens when we tried to inject event, >> injection was interrupted by other exception, and then we migrate? > > Ya, should Just Work if control.next_rip is used to cache the next rip. > > Handling this doesn't seem to be too awful (haven't tested yet), it's largely the > same logic as the existing !nrips code. > > In svm_update_soft_interrupt_rip(), snapshot all information regardless of whether > or not nrips is enabled: > > svm->soft_int_injected = true; > svm->soft_int_csbase = svm->vmcb->save.cs.base; > svm->soft_int_old_rip = old_rip; > svm->soft_int_next_rip = rip; > > if (nrips) > kvm_rip_write(vcpu, old_rip); > > if (static_cpu_has(X86_FEATURE_NRIPS)) > svm->vmcb->control.next_rip = rip; > > and then in svm_complete_interrupts(), change the linear RIP matching code to look > for the old rip in the nrips case and stuff svm->vmcb->control.next_rip on match. > > bool soft_int_injected = svm->soft_int_injected; > unsigned soft_int_rip; > > svm->soft_int_injected = false; > > if (soft_int_injected) { > if (nrips) > soft_int_rip = svm->soft_int_old_rip; > else > soft_int_rip = svm->soft_int_next_rip; > } > > ... > > if soft_int_injected && type == SVM_EXITINTINFO_TYPE_EXEPT && > kvm_is_linear_rip(vcpu, soft_int_rip + svm->soft_int_csbase)) { > if (nrips) > svm->vmcb->control.next_rip = svm->soft_int_next_rip; > else > kvm_rip_write(vcpu, svm->soft_int_old_rip); > } > > >
Despite what the svm_update_soft_interrupt_rip() name might suggest this handles only *soft exceptions*, not *soft interrupts* (which are injected by svm_inject_irq() and also need proper next_rip management).
Also, I'm not sure that even the proposed updated code above will actually restore the L1-requested next_rip correctly on L1 -> L2 re-injection (will review once the full version is available).
Thanks, Maciej
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