Re: [PATCH v3 2/2] xen/domain: rewrite emulation_flags_ok()

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 28 May 2025, dmkhn@xxxxxxxxx wrote:
> From: Denis Mukhin <dmkhn@xxxxxxxxx>
> 
> From: Denis Mukhin <dmukhin@xxxxxxxx>
> 
> Rewrite emulation_flags_ok() to simplify future modifications.
> 
> No functional change intended.
> 
> Signed-off-by: Denis Mukhin <dmukhin@xxxxxxxx>
> ---
> Changes since v2:
> - addressed review feedback
> - added some explanatory comments for emulation_flags_ok()
> ---
>  xen/arch/x86/domain.c | 92 ++++++++++++++++++++++++++++++++++---------
>  1 file changed, 74 insertions(+), 18 deletions(-)
> 
> diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
> index 0363ccb384..1d41d26c4d 100644
> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -743,32 +743,88 @@ int arch_sanitise_domain_config(struct 
> xen_domctl_createdomain *config)
>      return 0;
>  }
>  
> +/*
> + * Verify that the domain's emulation flags resolve to a supported 
> configuration.
> + *
> + * This ensures we only allow a known, safe subset of emulation combinations
> + * (for both functionality and security). Arbitrary mixes are likely to cause
> + * errors (e.g., null pointer dereferences).
> + *
> + * NB: use the internal X86_EMU_XXX symbols, not the public XEN_X86_EMU_XXX
> + * symbols.
> + */
>  static bool emulation_flags_ok(const struct domain *d, uint32_t emflags)
>  {
> +    enum {
> +        CAP_PV          = BIT(0, U),
> +        CAP_HVM         = BIT(1, U),
> +        CAP_HWDOM       = BIT(2, U),
> +        CAP_DOMU        = BIT(3, U),
> +    };
> +    static const struct {
> +        unsigned int caps;
> +        uint32_t min;
> +        uint32_t opt;
> +    } configs[] = {
> +#ifdef CONFIG_PV
> +        /* PV */
> +        {
> +            .caps   = CAP_PV | CAP_DOMU,
> +            .min    = 0,
> +            .opt    = 0,
> +        },
> +
> +        /* PV (likely dom0) */
> +        {
> +            .caps   = CAP_PV | CAP_HWDOM,
> +            .min    = X86_EMU_PIT,
> +            .opt    = 0,
> +        },
> +#endif /* #ifdef CONFIG_PV */
> +
> +#ifdef CONFIG_HVM
> +        /* PVH dom0/domU or HVM domU */
> +        {
> +            .caps   = CAP_HVM | CAP_HWDOM,
> +            .min    = X86_EMU_LAPIC | X86_EMU_IOAPIC | X86_EMU_VPCI,
> +            .opt    = 0,
> +        },
> +
> +

NIT: double \n

I checked carefully and the new semantic matches the old one with the
extra clarification that X86_EMU_PIT is only for hwdom.

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> +        /* HVM domU */
> +        {
> +            .caps   = CAP_HVM | CAP_DOMU,
> +            .min    = X86_EMU_ALL & ~(X86_EMU_VPCI | X86_EMU_USE_PIRQ),
> +            /* HVM PIRQ feature is user-selectable. */
> +            .opt    = X86_EMU_USE_PIRQ,
> +        },
> +
> +        /* PVH */
> +        {
> +            .caps   = CAP_HVM | CAP_DOMU,
> +            .min    = X86_EMU_LAPIC,
> +            .opt    = 0,
> +        },
> +#endif /* #ifdef CONFIG_HVM */
> +    };
> +    unsigned int i, caps = is_hardware_domain(d) ? CAP_HWDOM : CAP_DOMU;
> +
> +    if ( is_pv_domain(d) )
> +        caps |= CAP_PV;
> +    else if ( is_hvm_domain(d) )
> +        caps |= CAP_HVM;
> +
>  #ifdef CONFIG_HVM
>      /* This doesn't catch !CONFIG_HVM case but it is better than nothing */
>      BUILD_BUG_ON(X86_EMU_ALL != XEN_X86_EMU_ALL);
>  #endif
>  
> -    if ( is_hvm_domain(d) )
> -    {
> -        if ( is_hardware_domain(d) &&
> -             emflags != (X86_EMU_VPCI | X86_EMU_LAPIC | X86_EMU_IOAPIC) )
> -            return false;
> -        if ( !is_hardware_domain(d) &&
> -             /* HVM PIRQ feature is user-selectable. */
> -             (emflags & ~X86_EMU_USE_PIRQ) !=
> -             (X86_EMU_ALL & ~(X86_EMU_VPCI | X86_EMU_USE_PIRQ)) &&
> -             emflags != X86_EMU_LAPIC )
> -            return false;
> -    }
> -    else if ( emflags != 0 && emflags != X86_EMU_PIT )
> -    {
> -        /* PV or classic PVH. */
> -        return false;
> -    }
> +    for ( i = 0; i < ARRAY_SIZE(configs); i++ )
> +        if ( caps == configs[i].caps &&
> +             (emflags & ~configs[i].opt) == configs[i].min )
> +            return true;
>  
> -    return true;
> +    return false;
>  }
>  
>  void __init arch_init_idle_domain(struct domain *d)
> -- 
> 2.34.1
> 
>