linux-kernel - Re: [PATCH v6 1/7] KVM: x86: Deflect unknown MSR accesses to user space

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Date:   Thu, 3 Sep 2020 12:27:13 -0700
From:   Aaron Lewis <aaronlewis@...gle.com>
To:     Alexander Graf <graf@...zon.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        KarimAllah Raslan <karahmed@...zon.de>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        kvm list <kvm@...r.kernel.org>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/7] KVM: x86: Deflect unknown MSR accesses to user space

> +::
> +
> +               /* KVM_EXIT_X86_RDMSR / KVM_EXIT_X86_WRMSR */
> +               struct {
> +                       __u8 error; /* user -> kernel */
> +                       __u8 pad[3];
> +                       __u32 reason; /* kernel -> user */
> +                       __u32 index; /* kernel -> user */
> +                       __u64 data; /* kernel <-> user */
> +               } msr;
> +
> +Used on x86 systems. When the VM capability KVM_CAP_X86_USER_SPACE_MSR is
> +enabled, MSR accesses to registers that would invoke a #GP by KVM kernel code
> +will instead trigger a KVM_EXIT_X86_RDMSR exit for reads and KVM_EXIT_X86_WRMSR
> +exit for writes.
> +
> +The "reason" field specifies why the MSR trap occurred. User space will only
> +receive MSR exit traps when a particular reason was requested during through
> +ENABLE_CAP. Currently valid exit reasons are:
> +
> +       KVM_MSR_EXIT_REASON_INVAL - access to invalid MSRs or reserved bits


Can we also have ENOENT?
        KVM_MSR_EXIT_REASON_ENOENT - Unknown MSR

>
> +
> +For KVM_EXIT_X86_RDMSR, the "index" field tells user space which MSR the guest
> +wants to read. To respond to this request with a successful read, user space
> +writes the respective data into the "data" field and must continue guest
> +execution to ensure the read data is transferred into guest register state.
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 88c593f83b28..4d285bf054fb 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1549,12 +1549,88 @@ int kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data)
>  }
>  EXPORT_SYMBOL_GPL(kvm_set_msr);
>
> +static int complete_emulated_msr(struct kvm_vcpu *vcpu, bool is_read)
> +{
> +       if (vcpu->run->msr.error) {
> +               kvm_inject_gp(vcpu, 0);

Add return 1. The RIP doesn’t advance when the instruction raises a fault.

>
> +       } else if (is_read) {
> +               kvm_rax_write(vcpu, (u32)vcpu->run->msr.data);
> +               kvm_rdx_write(vcpu, vcpu->run->msr.data >> 32);
> +       }
> +
> +       return kvm_skip_emulated_instruction(vcpu);
> +}
> +
> +static int complete_emulated_rdmsr(struct kvm_vcpu *vcpu)
> +{
> +       return complete_emulated_msr(vcpu, true);
> +}
> +
>
>  /* For KVM_EXIT_INTERNAL_ERROR */
>  /* Emulate instruction failed. */
> @@ -412,6 +414,15 @@ struct kvm_run {
>                         __u64 esr_iss;
>                         __u64 fault_ipa;
>                 } arm_nisv;
> +               /* KVM_EXIT_X86_RDMSR / KVM_EXIT_X86_WRMSR */
> +               struct {
> +                       __u8 error; /* user -> kernel */
> +                       __u8 pad[3];

__u8 pad[7] to maintain 8 byte alignment?  unless we can get away with
fewer bits for 'reason' and
get them from 'pad'.

> +#define KVM_MSR_EXIT_REASON_INVAL      (1 << 0)
> +                       __u32 reason; /* kernel -> user */
> +                       __u32 index; /* kernel -> user */
> +                       __u64 data; /* kernel <-> user */
> +               } msr;
>                 /* Fix the size of the union. */
>                 char padding[256];
>         };
> @@ -1031,6 +1042,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_PPC_SECURE_GUEST 181
>  #define KVM_CAP_HALT_POLL 182
>  #define KVM_CAP_ASYNC_PF_INT 183
> +#define KVM_CAP_X86_USER_SPACE_MSR 184
>
>  #ifdef KVM_CAP_IRQ_ROUTING
>
> --
> 2.17.1
>
>
>
>
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>