Re: [PATCH v15 13/41] KVM: x86: Enable guest SSP read/write interface with new uAPIs

[Sean Christopherson <seanjc@google.com>]

On Fri, Sep 19, 2025, Sean Christopherson wrote:
> On Tue, Sep 16, 2025, Xiaoyao Li wrote:
> > On 9/16/2025 6:12 AM, Sean Christopherson wrote:
> > > For 6.18, I think the safe play is to go with the first path (exempt KVM-internal
> > > MSRs), and then try to go for the second approach (exempt all host accesses) for
> > > 6.19.  KVM's ABI for ignore_msrs=true is already all kinds of messed up, so I'm
> > > not terribly concerned about temporarily making it marginally worse.
> > 
> > Looks OK to me.
> 
> Actually, better idea.  Just use kvm_msr_{read,write}() for ONE_REG and bypass
> the ignore_msrs crud.  It's new uAPI, so we can define the semantics to be anything
> we want.  I see zero reason for ignore_msrs to apply to host accesses, and even
> less reason for it to apply to ONE_REG.
> 
> Then there's no need to special case GUEST_SSP, and what to do about ignore_msrs
> for host accesses remains an orthogonal discussion.
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4ed25d33aaee..4adfece25630 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5932,7 +5932,7 @@ static int kvm_get_one_msr(struct kvm_vcpu *vcpu, u32 msr, u64 __user *user_val)
>  {
>         u64 val;
>  
> -       if (do_get_msr(vcpu, msr, &val))
> +       if (kvm_msr_read(vcpu, msr, &val))
>                 return -EINVAL;
>  
>         if (put_user(val, user_val))
> @@ -5948,7 +5948,7 @@ static int kvm_set_one_msr(struct kvm_vcpu *vcpu, u32 msr, u64 __user *user_val)
>         if (get_user(val, user_val))
>                 return -EFAULT;
>  
> -       if (do_set_msr(vcpu, msr, &val))
> +       if (kvm_msr_write(vcpu, msr, &val))
>                 return -EINVAL;
>  
>         return 0;

Never mind, that would cause problems for using ONE_REG for actual MSRs.  Most
importantly, it would let userspace bypass the feature MSR restrictions in
do_set_msr().

I think the best option is to immediately reject translation.  That way host
accesses to whatever KVM uses for the internal SSP MSR index are unaffected by
the introduction of ONE_REG support.  E.g. modifying kvm_do_msr_access() would
mean that userspace would see different behavior for MSR_KVM_INTERNAL_GUEST_SSP
versus all other MSRs.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ab7f8c41d93b..720540f102e1 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6016,10 +6016,20 @@ struct kvm_x86_reg_id {
        __u8  x86;
 };
 
-static int kvm_translate_kvm_reg(struct kvm_x86_reg_id *reg)
+static int kvm_translate_kvm_reg(struct kvm_vcpu *vcpu,
+                                struct kvm_x86_reg_id *reg)
 {
        switch (reg->index) {
        case KVM_REG_GUEST_SSP:
+               /*
+                * FIXME: If host-initiated accesses are ever exempted from
+                * ignore_msrs (in kvm_do_msr_access()), drop this manual check
+                * and rely on KVM's standard checks to reject accesses to regs
+                * that don't exist.
+                */
+               if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK))
+                       return -EINVAL;
+
                reg->type = KVM_X86_REG_TYPE_MSR;
                reg->index = MSR_KVM_INTERNAL_GUEST_SSP;
                break;
@@ -6075,7 +6085,7 @@ static int kvm_get_set_one_reg(struct kvm_vcpu *vcpu, unsigned int ioctl,
                return -EINVAL;
 
        if (reg->type == KVM_X86_REG_TYPE_KVM) {
-               r = kvm_translate_kvm_reg(reg);
+               r = kvm_translate_kvm_reg(vcpu, reg);
                if (r)
                        return r;
        }