Re: [PATCH 1/8] x86/msr: Fix migration compatibility issue with MSR_SPEC_CTRL

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Jan 26, 2022 at 08:44:45AM +0000, Andrew Cooper wrote:
> This bug existed in early in 2018 between MSR_SPEC_CTRL arriving in microcode,
> and SSBD arriving a few months later.  It went unnoticed presumably because
> everyone was busy rebooting everything.
> 
> The same bug will reappear when adding PSFD support.
> 
> Clamp the guest MSR_SPEC_CTRL value to that permitted by CPUID on migrate.
> The guest is already playing with reserved bits at this point, and clamping
> the value will prevent a migration to a less capable host from failing.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Wei Liu <wl@xxxxxxx>
> ---
>  xen/arch/x86/hvm/hvm.c         | 25 +++++++++++++++++++++++--
>  xen/arch/x86/include/asm/msr.h |  2 ++
>  xen/arch/x86/msr.c             | 33 +++++++++++++++++++++------------
>  3 files changed, 46 insertions(+), 14 deletions(-)
> 
> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> index d7d3299b431e..c4ddb8607d9c 100644
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -1340,6 +1340,7 @@ static const uint32_t msrs_to_send[] = {
>  
>  static int hvm_save_cpu_msrs(struct vcpu *v, hvm_domain_context_t *h)
>  {
> +    const struct domain *d = v->domain;
>      struct hvm_save_descriptor *desc = _p(&h->data[h->cur]);
>      struct hvm_msr *ctxt;
>      unsigned int i;
> @@ -1355,7 +1356,8 @@ static int hvm_save_cpu_msrs(struct vcpu *v, 
> hvm_domain_context_t *h)
>      for ( i = 0; i < ARRAY_SIZE(msrs_to_send); ++i )
>      {
>          uint64_t val;
> -        int rc = guest_rdmsr(v, msrs_to_send[i], &val);
> +        unsigned int msr = msrs_to_send[i];
> +        int rc = guest_rdmsr(v, msr, &val);
>  
>          /*
>           * It is the programmers responsibility to ensure that
> @@ -1375,7 +1377,26 @@ static int hvm_save_cpu_msrs(struct vcpu *v, 
> hvm_domain_context_t *h)
>          if ( !val )
>              continue; /* Skip empty MSRs. */
>  
> -        ctxt->msr[ctxt->count].index = msrs_to_send[i];
> +        /*
> +         * Guests are given full access to certain MSRs for performance
> +         * reasons.  A consequence is that Xen is unable to enforce that all
> +         * bits disallowed by the CPUID policy yield #GP, and an enterprising
> +         * guest may be able to set and use a bit it ought to leave alone.
> +         *
> +         * When migrating from a more capable host to a less capable one, 
> such
> +         * bits may be rejected by the destination, and the migration failed.
> +         *
> +         * Discard such bits here on the source side.  Such bits have 
> reserved
> +         * behaviour, and the guest has only itself to blame.
> +         */
> +        switch ( msr )
> +        {
> +        case MSR_SPEC_CTRL:
> +            val &= msr_spec_ctrl_valid_bits(d->arch.cpuid);
> +            break;
> +        }

Should you move the check for !val here, in case the clearing done
here zeros the MSR?

> +
> +        ctxt->msr[ctxt->count].index = msr;
>          ctxt->msr[ctxt->count++].val = val;
>      }
>  
> diff --git a/xen/arch/x86/include/asm/msr.h b/xen/arch/x86/include/asm/msr.h
> index 10039c2d227b..657a3295613d 100644
> --- a/xen/arch/x86/include/asm/msr.h
> +++ b/xen/arch/x86/include/asm/msr.h
> @@ -277,6 +277,8 @@ static inline void wrmsr_tsc_aux(uint32_t val)
>      }
>  }
>  
> +uint64_t msr_spec_ctrl_valid_bits(const struct cpuid_policy *cp);
> +
>  extern struct msr_policy     raw_msr_policy,
>                              host_msr_policy,
>                            pv_max_msr_policy,
> diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
> index 2cc355575d45..5e80c8b47c21 100644
> --- a/xen/arch/x86/msr.c
> +++ b/xen/arch/x86/msr.c
> @@ -435,6 +435,24 @@ int guest_rdmsr(struct vcpu *v, uint32_t msr, uint64_t 
> *val)
>      return X86EMUL_EXCEPTION;
>  }
>  
> +/*
> + * Caller to confirm that MSR_SPEC_CTRL is available.  Intel and AMD have
> + * separate CPUID features for this functionality, but only set will be
> + * active.
> + */
> +uint64_t msr_spec_ctrl_valid_bits(const struct cpuid_policy *cp)
> +{
> +    bool ssbd = cp->feat.ssbd;
> +
> +    /*
> +     * Note: SPEC_CTRL_STIBP is specified as safe to use (i.e. ignored)
> +     * when STIBP isn't enumerated in hardware.
> +     */
> +    return (SPEC_CTRL_IBRS | SPEC_CTRL_STIBP |
> +            (ssbd       ? SPEC_CTRL_SSBD       : 0) |
> +            0);

The format here looks weird to me, and I don't get why you
unconditionally or a 0 at the end?

I would also be fine with using cp->feat.ssbd directly here.

Thanks, Roger.