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SubjectRe: [PATCH 4/6] KVM: nSVM: Fix L1 state corruption upon return from SMM
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On Mon, 2021-06-28 at 12:44 +0200, Vitaly Kuznetsov wrote:
> VMCB split commit 4995a3685f1b ("KVM: SVM: Use a separate vmcb for the
> nested L2 guest") broke return from SMM when we entered there from guest
> (L2) mode. Gen2 WS2016/Hyper-V is known to do this on boot. The problem
> manifests itself like this:
>
> kvm_exit: reason EXIT_RSM rip 0x7ffbb280 info 0 0
> kvm_emulate_insn: 0:7ffbb280: 0f aa
> kvm_smm_transition: vcpu 0: leaving SMM, smbase 0x7ffb3000
> kvm_nested_vmrun: rip: 0x000000007ffbb280 vmcb: 0x0000000008224000
> nrip: 0xffffffffffbbe119 int_ctl: 0x01020000 event_inj: 0x00000000
> npt: on
> kvm_nested_intercepts: cr_read: 0000 cr_write: 0010 excp: 40060002
> intercepts: fd44bfeb 0000217f 00000000
> kvm_entry: vcpu 0, rip 0xffffffffffbbe119
> kvm_exit: reason EXIT_NPF rip 0xffffffffffbbe119 info
> 200000006 1ab000
> kvm_nested_vmexit: vcpu 0 reason npf rip 0xffffffffffbbe119 info1
> 0x0000000200000006 info2 0x00000000001ab000 intr_info 0x00000000
> error_code 0x00000000
> kvm_page_fault: address 1ab000 error_code 6
> kvm_nested_vmexit_inject: reason EXIT_NPF info1 200000006 info2 1ab000
> int_info 0 int_info_err 0
> kvm_entry: vcpu 0, rip 0x7ffbb280
> kvm_exit: reason EXIT_EXCP_GP rip 0x7ffbb280 info 0 0
> kvm_emulate_insn: 0:7ffbb280: 0f aa
> kvm_inj_exception: #GP (0x0)
>
> Note: return to L2 succeeded but upon first exit to L1 its RIP points to
> 'RSM' instruction but we're not in SMM.
>
> The problem appears to be that VMCB01 gets irreversibly destroyed during
> SMM execution. Previously, we used to have 'hsave' VMCB where regular
> (pre-SMM) L1's state was saved upon nested_svm_vmexit() but now we just
> switch to VMCB01 from VMCB02.
>
> Pre-split (working) flow looked like:
> - SMM is triggered during L2's execution
> - L2's state is pushed to SMRAM
> - nested_svm_vmexit() restores L1's state from 'hsave'
> - SMM -> RSM
> - enter_svm_guest_mode() switches to L2 but keeps 'hsave' intact so we have
> pre-SMM (and pre L2 VMRUN) L1's state there
> - L2's state is restored from SMRAM
> - upon first exit L1's state is restored from L1.
>
> This was always broken with regards to svm_get_nested_state()/
> svm_set_nested_state(): 'hsave' was never a part of what's being
> save and restored so migration happening during SMM triggered from L2 would
> never restore L1's state correctly.
>
> Post-split flow (broken) looks like:
> - SMM is triggered during L2's execution
> - L2's state is pushed to SMRAM
> - nested_svm_vmexit() switches to VMCB01 from VMCB02
> - SMM -> RSM
> - enter_svm_guest_mode() switches from VMCB01 to VMCB02 but pre-SMM VMCB01
> is already lost.
> - L2's state is restored from SMRAM
> - upon first exit L1's state is restored from VMCB01 but it is corrupted
> (reflects the state during 'RSM' execution).
>
> VMX doesn't have this problem because unlike VMCB, VMCS keeps both guest
> and host state so when we switch back to VMCS02 L1's state is intact there.
>
> To resolve the issue we need to save L1's state somewhere. We could've
> created a third VMCB for SMM but that would require us to modify saved
> state format. L1's architectural HSAVE area (pointed by MSR_VM_HSAVE_PA)
> seems appropriate: L0 is free to save any (or none) of L1's state there.
> Currently, KVM does 'none'.
>
> Note, for nested state migration to succeed, both source and destination
> hypervisors must have the fix. We, however, don't need to create a new
> flag indicating the fact that HSAVE area is now populated as migration
> during SMM triggered from L2 was always broken.
>
> Fixes: 4995a3685f1b ("KVM: SVM: Use a separate vmcb for the nested L2 guest")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> - RFC: I'm not 100% sure my 'smart' idea to use currently-unused HSAVE area
> is that smart. Also, we don't even seem to check that L1 set it up upon
> nested VMRUN so hypervisors which don't do that may remain broken. A very
> much needed selftest is also missing.
> ---
> arch/x86/kvm/svm/svm.c | 39 ++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 38 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index b6f85fd19f96..fbf1b352a9bb 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4291,6 +4291,7 @@ static int svm_smi_allowed(struct kvm_vcpu *vcpu, bool for_injection)
> static int svm_enter_smm(struct kvm_vcpu *vcpu, char *smstate)
> {
> struct vcpu_svm *svm = to_svm(vcpu);
> + struct kvm_host_map map_save;
> int ret;
>
> if (is_guest_mode(vcpu)) {
> @@ -4306,6 +4307,29 @@ static int svm_enter_smm(struct kvm_vcpu *vcpu, char *smstate)
> ret = nested_svm_vmexit(svm);
> if (ret)
> return ret;
> +
> + /*
> + * KVM uses VMCB01 to cache L1 host state while L2 runs but
> + * VMCB01 is going to be used during SMM and thus the state will
> + * be lost. Temporary save non-VMLOAD/VMSAVE state to host save
> + * area pointed to by MSR_VM_HSAVE_PA. APM guarantees that the
> + * format of the area is identical to guest save area offsetted
> + * by 0x400 (matches the offset of 'struct vmcb_save_area'
> + * within 'struct vmcb'). Note: HSAVE area may also be used by
> + * L1 hypervisor to save additional host context (e.g. KVM does
> + * that, see svm_prepare_guest_switch()) which must be
> + * preserved.
> + */

Minor nitpick: I woudn't say that KVM "caches" L1 host state as this implies
that there is a master copy somewhere.
I would write something like that instead:

"KVM stores L1 host state in VMCB01 because the CPU naturally stores it there."

Other than that the comment looks very good.



> + if (kvm_vcpu_map(vcpu, gpa_to_gfn(svm->nested.hsave_msr),
> + &map_save) == -EINVAL)
> + return 1;

Looks like the return value of vendor specific 'enter_smm' isn't checked
in the common 'enter_smm' handler which itself returns 'void', and doesn't
check anything it does either.

enter_smm is called from inject_pending_event which is allowed to return
a negative value which is assumed to be -EBUSY and doesn't lead
to exit with negative value to userspace.
I vote to fix this and only hide -EBUSY from the userspace,
and let all other errors from this function end up in userspace
so that userspace could kill the guest.

This isn't this patch fault though, and I think I'll send a patch
to do the above.


> +
> + BUILD_BUG_ON(offsetof(struct vmcb, save) != 0x400);
I doubt that this will ever change, but let it be.
> +
> + svm_copy_nonvmloadsave_state(&svm->vmcb01.ptr->save,
> + map_save.hva + 0x400);
> +
> + kvm_vcpu_unmap(vcpu, &map_save, true);
> }
> return 0;
> }
> @@ -4313,7 +4337,7 @@ static int svm_enter_smm(struct kvm_vcpu *vcpu, char *smstate)
> static int svm_leave_smm(struct kvm_vcpu *vcpu, const char *smstate)
> {
> struct vcpu_svm *svm = to_svm(vcpu);
> - struct kvm_host_map map;
> + struct kvm_host_map map, map_save;
> int ret = 0;
>
> if (guest_cpuid_has(vcpu, X86_FEATURE_LM)) {
> @@ -4337,6 +4361,19 @@ static int svm_leave_smm(struct kvm_vcpu *vcpu, const char *smstate)
>
> ret = enter_svm_guest_mode(vcpu, vmcb12_gpa, map.hva);
> kvm_vcpu_unmap(vcpu, &map, true);
> +
> + /*
> + * Restore L1 host state from L1 HSAVE area as VMCB01 was
> + * used during SMM (see svm_enter_smm())
> + */
Looks fine as well.

> + if (kvm_vcpu_map(vcpu, gpa_to_gfn(svm->nested.hsave_msr),
> + &map_save) == -EINVAL)
> + return 1;
> +
> + svm_copy_nonvmloadsave_state(map_save.hva + 0x400,
> + &svm->vmcb01.ptr->save);
> +
> + kvm_vcpu_unmap(vcpu, &map_save, true);
> }
> }
>

So besides the nitpick of the comment,
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
Maxim Levitsky

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