Messages in this thread | | | Date | Wed, 23 Aug 2023 15:36:25 -0500 | Subject | Re: [PATCH 0/2] KVM: SVM: Fix unexpected #UD on INT3 in SEV guests | From | Tom Lendacky <> |
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On 8/23/23 15:03, Sean Christopherson wrote: > On Wed, Aug 23, 2023, Tom Lendacky wrote: >> On 8/22/23 10:14, Sean Christopherson wrote: >>> On Tue, Aug 22, 2023, Tom Lendacky wrote: >>>> On 8/10/23 18:49, Sean Christopherson wrote: >>>>> Fix a bug where KVM injects a bogus #UD for SEV guests when trying to skip >>>>> an INT3 as part of re-injecting the associated #BP that got kinda sorta >>>>> intercepted due to a #NPF occuring while vectoring/delivering the #BP. >>>>> >>>>> I haven't actually confirmed that patch 1 fixes the bug, as it's a >>>>> different change than what I originally proposed. I'm 99% certain it will >>>>> work, but I definitely need verification that it fixes the problem >>>>> >>>>> Patch 2 is a tangentially related cleanup to make NRIPS a requirement for >>>>> enabling SEV, e.g. so that we don't ever get "bug" reports of SEV guests >>>>> not working when NRIPS is disabled. >>>>> >>>>> Sean Christopherson (2): >>>>> KVM: SVM: Don't inject #UD if KVM attempts emulation of SEV guest w/o >>>>> insn >>>>> KVM: SVM: Require nrips support for SEV guests (and beyond) >>>>> >>>>> arch/x86/kvm/svm/sev.c | 2 +- >>>>> arch/x86/kvm/svm/svm.c | 37 ++++++++++++++++++++----------------- >>>>> arch/x86/kvm/svm/svm.h | 1 + >>>>> 3 files changed, 22 insertions(+), 18 deletions(-) >>>> >>>> We ran some stress tests against a version of the kernel without this fix >>>> and we're able to reproduce the issue, but not reliably, after a few hours. >>>> With this patch, it has not reproduced after running for a week. >>>> >>>> Not as reliable a scenario as the original reporter, but this looks like it >>>> resolves the issue. >>> >>> Thanks Tom! I'll apply this for v6.6, that'll give us plenty of time to change >>> course if necessary. >> >> I may have spoke to soon... When the #UD was triggered it was here: >> >> [ 0.118524] Spectre V2 : Enabling Restricted Speculation for firmware calls >> [ 0.118524] Spectre V2 : mitigation: Enabling conditional Indirect Branch Prediction Barrier >> [ 0.118524] Speculative Store Bypass: Mitigation: Speculative Store Bypass disabled via prctl >> [ 0.118524] invalid opcode: 0000 [#1] PREEMPT SMP NOPTI >> [ 0.118524] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.2.2-amdsos-build50-ubuntu-20.04+ #1 >> [ 0.118524] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 >> [ 0.118524] RIP: 0010:int3_selftest_ip+0x0/0x60 >> [ 0.118524] Code: b9 25 05 00 00 48 c7 c2 e8 7c 80 b0 48 c7 c6 fe 1c d3 b0 48 c7 c7 f0 7d da b0 e8 4c 2c 0b ff e8 75 da 15 ff 0f 0b 48 8d 7d f4 <cc> 90 90 90 90 83 7d f4 01 74 2f 80 3d 39 7f a8 00 00 74 24 b9 34 >> >> >> Now (after about a week) we've encountered a hang here: >> >> [ 0.106216] Spectre V2 : Enabling Restricted Speculation for firmware calls >> [ 0.106216] Spectre V2 : mitigation: Enabling conditional Indirect Branch Prediction Barrier >> [ 0.106216] Speculative Store Bypass: Mitigation: Speculative Store Bypass disabled via prctl >> >> It is in the very same spot and so I wonder if the return false (without >> queuing a #UD) is causing an infinite loop here that appears as a guest >> hang. Whereas, we have some systems running the first patch that you >> created that have not hit this hang. >> >> But I'm not sure why or how this patch could cause the guest hang. I >> would think that the retry of the instruction would resolve everything >> and the guest would continue. Unfortunately, the guest was killed, so I'll >> try to reproduce and get a dump or trace points of the VM to see what is >> going on. > > Gah, it's because x86_emulate_instruction() returns '1' and not '0' when > svm_can_emulate_instruction() returns false. svm_update_soft_interrupt_rip() > would then continue with the injection, i.e. inject #BP on the INT3 RIP, not on > the RIP following the INT3, which would cause this check to fail > > if (regs->ip - INT3_INSN_SIZE != selftest) > return NOTIFY_DONE; > > and eventually send do_trap_no_signal() to die().
Thanks for figuring that out so quickly!
> > I distinctly remember seeing the return value problem when writing the patch, but > missed that it would result in KVM injecting the unexpected #BP. > > I punted on trying to properly fix this by having can_emulate_instruction() > differentiate between "retry insn" and "inject exception", because that change > is painfully invasive and I though I could get away with the simple fix. Drat. > > I think the best option is to add a "temporary" patch so that the fix for @stable > is short, sweet, and safe, and then do the can_emulate_instruction() cleanup that > I was avoiding. > > E.g. this as patch 2/4 (or maybe 2/5) of this series:
2/4 or 2/5? Do you mean 2/3 since there were only 2 patches in the series?
I'll apply the below patch in between patches 1 and 2 and re-test. Should have results in a week :)
Thanks, Tom
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c > index 7cb5ef5835c2..8457a36b44c1 100644 > --- a/arch/x86/kvm/svm/svm.c > +++ b/arch/x86/kvm/svm/svm.c > @@ -364,6 +364,8 @@ static void svm_set_interrupt_shadow(struct kvm_vcpu *vcpu, int mask) > svm->vmcb->control.int_state |= SVM_INTERRUPT_SHADOW_MASK; > > } > +static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type, > + void *insn, int insn_len); > > static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu, > bool commit_side_effects) > @@ -384,6 +386,14 @@ static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu, > } > > if (!svm->next_rip) { > + /* > + * FIXME: Drop this when kvm_emulate_instruction() does the > + * right thing and treats "can't emulate" as outright failure > + * for EMULTYPE_SKIP. > + */ > + if (!svm_can_emulate_instruction(vcpu, EMULTYPE_SKIP, NULL, 0)) > + return 0; > + > if (unlikely(!commit_side_effects)) > old_rflags = svm->vmcb->save.rflags;
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