Re: [PATCH v6] xen/domain: rewrite emulation_flags_ok()

["Teddy Astie" <teddy.astie@xxxxxxxxxx>]

Hello,

Le 10/06/2025 à 02:45, dmkhn@xxxxxxxxx a écrit :
> From: Denis Mukhin <dmukhin@xxxxxxxx>
>
> Rewrite emulation_flags_ok() to simplify future modifications.
>
> No functional change intended.
>
> Signed-off-by: Denis Mukhin <dmukhin@xxxxxxxx>

Reviewed-by: Teddy Astie <teddy.astie@xxxxxxxxxx>

> ---
> Changes since v5:
> - optimized `configs[]` table - just one record for PV case
> - sorted entries in `configs[]` table by domain type: PV, then PVH, then HVM
>    entries
> - addressed `caps` initializaton
>
> Link to v5: 
> https://lore.kernel.org/xen-devel/20250602191717.148361-3-dmukhin@xxxxxxxx/
> Link to CI: 
> https://gitlab.com/xen-project/people/dmukhin/xen/-/pipelines/1861382846/
> ---
>   xen/arch/x86/domain.c | 86 ++++++++++++++++++++++++++++++++++---------
>   1 file changed, 68 insertions(+), 18 deletions(-)
>
> diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
> index 7536b6c871..82b126351b 100644
> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -743,32 +743,82 @@ 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 dom0 and domU */
> +        {
> +            .caps   = CAP_PV | CAP_HWDOM | CAP_DOMU,
> +            .min    = X86_EMU_PIT,
> +        },
> +#endif /* #ifdef CONFIG_PV */
> +
> +#ifdef CONFIG_HVM
> +        /* PVH dom0 */
> +        {
> +            .caps   = CAP_HVM | CAP_HWDOM,
> +            .min    = X86_EMU_LAPIC | X86_EMU_IOAPIC | X86_EMU_VPCI,
> +        },
> +
> +        /* PVH domU */
> +        {
> +            .caps   = CAP_HVM | CAP_DOMU,
> +            .min    = X86_EMU_LAPIC,
> +        },
> +
> +        /* 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,
> +        },
> +#endif /* #ifdef CONFIG_HVM */
> +    };
> +    unsigned int i;
> +    unsigned int caps = (is_pv_domain(d) ? CAP_PV : CAP_HVM) |
> +                        (is_hardware_domain(d) ? CAP_HWDOM : CAP_DOMU);
> +
> +    /*
> +     * NB: PV domain can have 0 in emulation_flags.
> +     * See qemu-alpine-x86_64-gcc CI job.
> +     * Inject fake flag to keep the code checks simple.
> +     */
> +    if ( (caps & CAP_PV) && emflags == 0 )
> +        emflags |= X86_EMU_PIT;
> +
>   #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) == caps &&
> +             (emflags & ~configs[i].opt) == configs[i].min )
> +            return true;
>
> -    return true;
> +    return false;
>   }
>
>   void __init arch_init_idle_domain(struct domain *d)


Teddy Astie | Vates XCP-ng Developer

XCP-ng & Xen Orchestra - Vates solutions

web: https://vates.tech