Re: [PATCH v3 1/2] xsm: create idle domain privileged and demote after setup

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

On Fri, Apr 22, 2022 at 12:34:57PM -0400, Daniel P. Smith wrote:
> There are now instances where internal hypervisor logic needs to make resource
> allocation calls that are protected by XSM checks. The internal hypervisor 
> logic
> is represented a number of system domains which by designed are represented by

'Some of the hypervisor code can be executed in a system domain that's
represented by a per-CPU non-privileged struct domain. To enable...'

> non-privileged struct domain instances. To enable these logic blocks to
> function correctly but in a controlled manner, this commit changes the idle
> domain to be created as a privileged domain under the default policy, which is
> inherited by the SILO policy, and demoted before transitioning to running. A
> new XSM hook, xsm_set_system_active, is introduced to allow each XSM policy
> type to demote the idle domain appropriately for that policy type.
> 
> For flask a stub is added to ensure that flask policy system will function
> correctly with this patch until flask is extended with support for starting 
> the
> idle domain privileged and properly demoting it on the call to
> xsm_set_system_active.
> 
> Signed-off-by: Daniel P. Smith <dpsmith@xxxxxxxxxxxxxxxxxxxx>
> ---
>  xen/arch/arm/setup.c    |  3 +++
>  xen/arch/x86/setup.c    |  3 +++
>  xen/common/sched/core.c |  7 ++++++-
>  xen/include/xsm/dummy.h | 17 +++++++++++++++++
>  xen/include/xsm/xsm.h   |  6 ++++++
>  xen/xsm/dummy.c         |  1 +
>  xen/xsm/flask/hooks.c   | 21 +++++++++++++++++++++
>  7 files changed, 57 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
> index d5d0792ed4..e71fa3f860 100644
> --- a/xen/arch/arm/setup.c
> +++ b/xen/arch/arm/setup.c
> @@ -1048,6 +1048,9 @@ void __init start_xen(unsigned long boot_phys_offset,
>      /* Hide UART from DOM0 if we're using it */
>      serial_endboot();
>  
> +    if ( xsm_set_system_active() != 0)
> +        panic("xsm: unable to set hypervisor to SYSTEM_ACTIVE privilege\n");
> +
>      system_state = SYS_STATE_active;
>  
>      for_each_domain( d )
> diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
> index 6f20e17892..a3ce288ef9 100644
> --- a/xen/arch/x86/setup.c
> +++ b/xen/arch/x86/setup.c
> @@ -621,6 +621,9 @@ static void noreturn init_done(void)
>      void *va;
>      unsigned long start, end;
>  
> +    if ( xsm_set_system_active() != 0)
           ^ extra space.

Since the function returns an error code you might as well add it to
the panic message, or else just make the function return bool instead.

Or just make the function void and panic in the handler itself (like
in previous versions), as I don't think it's sensible to continue
normal execution if xsm_set_system_active fails.

> +        panic("xsm: unable to set hypervisor to SYSTEM_ACTIVE privilege\n");
> +
>      system_state = SYS_STATE_active;
>  
>      domain_unpause_by_systemcontroller(dom0);
> diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
> index 19ab678181..22a619e260 100644
> --- a/xen/common/sched/core.c
> +++ b/xen/common/sched/core.c
> @@ -3021,7 +3021,12 @@ void __init scheduler_init(void)
>          sched_ratelimit_us = SCHED_DEFAULT_RATELIMIT_US;
>      }
>  
> -    idle_domain = domain_create(DOMID_IDLE, NULL, 0);
> +    /*
> +     * idle dom is created privileged to ensure unrestricted access during
> +     * setup and will be demoted by xsm_transition_running when setup is

s/xsm_transition_running/xsm_set_system_active/

> +     * complete

Nit: missing full stop according to CODING_STYLE.

> +     */
> +    idle_domain = domain_create(DOMID_IDLE, NULL, CDF_privileged);
>      BUG_ON(IS_ERR(idle_domain));
>      BUG_ON(nr_cpu_ids > ARRAY_SIZE(idle_vcpu));
>      idle_domain->vcpu = idle_vcpu;
> diff --git a/xen/include/xsm/dummy.h b/xen/include/xsm/dummy.h
> index 58afc1d589..3291fb5396 100644
> --- a/xen/include/xsm/dummy.h
> +++ b/xen/include/xsm/dummy.h
> @@ -101,6 +101,23 @@ static always_inline int xsm_default_action(
>      }
>  }
>  
> +static XSM_INLINE int cf_check xsm_set_system_active(void)
> +{
> +    struct domain *d = current->domain;
> +
> +    ASSERT(d->is_privileged);
> +
> +    if ( d->domain_id != DOMID_IDLE )
> +    {
> +        printk("xsm_set_system_active: should only be called by idle 
> domain\n");
> +        return -EPERM;
> +    }
> +
> +    d->is_privileged = false;
> +
> +    return 0;
> +}
> +
>  static XSM_INLINE void cf_check xsm_security_domaininfo(
>      struct domain *d, struct xen_domctl_getdomaininfo *info)
>  {
> diff --git a/xen/include/xsm/xsm.h b/xen/include/xsm/xsm.h
> index 3e2b7fe3db..8dad03fd3d 100644
> --- a/xen/include/xsm/xsm.h
> +++ b/xen/include/xsm/xsm.h
> @@ -52,6 +52,7 @@ typedef enum xsm_default xsm_default_t;
>   * !!! WARNING !!!
>   */
>  struct xsm_ops {
> +    int (*set_system_active)(void);
>      void (*security_domaininfo)(struct domain *d,
>                                  struct xen_domctl_getdomaininfo *info);
>      int (*domain_create)(struct domain *d, uint32_t ssidref);
> @@ -208,6 +209,11 @@ extern struct xsm_ops xsm_ops;
>  
>  #ifndef XSM_NO_WRAPPERS
>  
> +static inline int xsm_set_system_active(void)
> +{
> +    return alternative_call(xsm_ops.set_system_active);
> +}
> +
>  static inline void xsm_security_domaininfo(
>      struct domain *d, struct xen_domctl_getdomaininfo *info)
>  {
> diff --git a/xen/xsm/dummy.c b/xen/xsm/dummy.c
> index 8c044ef615..e6ffa948f7 100644
> --- a/xen/xsm/dummy.c
> +++ b/xen/xsm/dummy.c
> @@ -14,6 +14,7 @@
>  #include <xsm/dummy.h>
>  
>  static const struct xsm_ops __initconst_cf_clobber dummy_ops = {
> +    .set_system_active             = xsm_set_system_active,
>      .security_domaininfo           = xsm_security_domaininfo,
>      .domain_create                 = xsm_domain_create,
>      .getdomaininfo                 = xsm_getdomaininfo,
> diff --git a/xen/xsm/flask/hooks.c b/xen/xsm/flask/hooks.c
> index 0bf63ffa84..8a62de2fd6 100644
> --- a/xen/xsm/flask/hooks.c
> +++ b/xen/xsm/flask/hooks.c
> @@ -186,6 +186,26 @@ static int cf_check flask_domain_alloc_security(struct 
> domain *d)
>      return 0;
>  }
>  
> +static int cf_check flask_set_system_active(void)
> +{
> +    struct domain *d = current->domain;

Nit: you should also add the assert for d->is_privileged, I don't see
a reason for the xsm and flask functions to differ in that regard.

Thanks, Roger.