RE: [PATCH 4/5] xen/memory: Fix acquire_resource size semantics

[Paul Durrant <xadimgnik@gmail.com>]

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@lists.xenproject.org> On Behalf Of Andrew Cooper
> Sent: 28 July 2020 12:37
> To: Xen-devel <xen-devel@lists.xenproject.org>
> Cc: Hubert Jasudowicz <hubert.jasudowicz@cert.pl>; Stefano Stabellini <sstabellini@kernel.org>; Julien
> Grall <julien@xen.org>; Wei Liu <wl@xen.org>; Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>; George
> Dunlap <George.Dunlap@eu.citrix.com>; Andrew Cooper <andrew.cooper3@citrix.com>; Paul Durrant
> <paul@xen.org>; Jan Beulich <JBeulich@suse.com>; Michał Leszczyński <michal.leszczynski@cert.pl>; Ian
> Jackson <ian.jackson@citrix.com>
> Subject: [PATCH 4/5] xen/memory: Fix acquire_resource size semantics
> 
> Calling XENMEM_acquire_resource with a NULL frame_list is a request for the
> size of the resource, but the returned 32 is bogus.
> 
> If someone tries to follow it for XENMEM_resource_ioreq_server, the acquire
> call will fail as IOREQ servers currently top out at 2 frames, and it is only
> half the size of the default grant table limit for guests.
> 
> Also, no users actually request a resource size, because it was never wired up
> in the sole implemenation of resource aquisition in Linux.
> 
> Introduce a new resource_max_frames() to calculate the size of a resource, and
> implement it the IOREQ and grant subsystems.
> 
> It is impossible to guarentee that a mapping call following a successful size

s/guarantee/guarantee

> call will succedd (e.g. The target IOREQ server gets destroyed, or the domain

s/succedd/succeed

> switches from grant v2 to v1).  Document the restriction, and use the
> flexibility to simplify the paths to be lockless.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
> CC: George Dunlap <George.Dunlap@eu.citrix.com>
> CC: Ian Jackson <ian.jackson@citrix.com>
> CC: Jan Beulich <JBeulich@suse.com>
> CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> CC: Stefano Stabellini <sstabellini@kernel.org>
> CC: Wei Liu <wl@xen.org>
> CC: Julien Grall <julien@xen.org>
> CC: Paul Durrant <paul@xen.org>
> CC: Michał Leszczyński <michal.leszczynski@cert.pl>
> CC: Hubert Jasudowicz <hubert.jasudowicz@cert.pl>
> ---
>  xen/arch/x86/mm.c             | 20 ++++++++++++++++
>  xen/common/grant_table.c      | 19 +++++++++++++++
>  xen/common/memory.c           | 55 +++++++++++++++++++++++++++++++++----------
>  xen/include/asm-x86/mm.h      |  3 +++
>  xen/include/public/memory.h   | 16 +++++++++----
>  xen/include/xen/grant_table.h |  8 +++++++
>  xen/include/xen/mm.h          |  6 +++++
>  7 files changed, 110 insertions(+), 17 deletions(-)
> 
> diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
> index 82bc676553..f73a90a2ab 100644
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -4600,6 +4600,26 @@ int xenmem_add_to_physmap_one(
>      return rc;
>  }
> 
> +unsigned int arch_resource_max_frames(
> +    struct domain *d, unsigned int type, unsigned int id)
> +{
> +    unsigned int nr = 0;
> +
> +    switch ( type )
> +    {
> +#ifdef CONFIG_HVM
> +    case XENMEM_resource_ioreq_server:
> +        if ( !is_hvm_domain(d) )
> +            break;
> +        /* One frame for the buf-ioreq ring, and one frame per 128 vcpus. */
> +        nr = 1 + DIV_ROUND_UP(d->max_vcpus * sizeof(struct ioreq), PAGE_SIZE);
> +        break;
> +#endif
> +    }
> +
> +    return nr;
> +}
> +
>  int arch_acquire_resource(struct domain *d, unsigned int type,
>                            unsigned int id, unsigned long frame,
>                            unsigned int nr_frames, xen_pfn_t mfn_list[])
> diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
> index 122d1e7596..0962fc7169 100644
> --- a/xen/common/grant_table.c
> +++ b/xen/common/grant_table.c
> @@ -4013,6 +4013,25 @@ static int gnttab_get_shared_frame_mfn(struct domain *d,
>      return 0;
>  }
> 
> +unsigned int gnttab_resource_max_frames(struct domain *d, unsigned int id)
> +{
> +    unsigned int nr = 0;
> +
> +    /* Don't need the grant lock.  This limit is fixed at domain create time. */
> +    switch ( id )
> +    {
> +    case XENMEM_resource_grant_table_id_shared:
> +        nr = d->grant_table->max_grant_frames;
> +        break;
> +
> +    case XENMEM_resource_grant_table_id_status:
> +        nr = grant_to_status_frames(d->grant_table->max_grant_frames);

Two uses of d->grant_table, so perhaps define a stack variable for it? Also, should you not make sure 0 is returned in the case of a v1 table?

> +        break;
> +    }
> +
> +    return nr;
> +}
> +
>  int gnttab_acquire_resource(
>      struct domain *d, unsigned int id, unsigned long frame,
>      unsigned int nr_frames, xen_pfn_t mfn_list[])
> diff --git a/xen/common/memory.c b/xen/common/memory.c
> index dc3a7248e3..21edabf9cc 100644
> --- a/xen/common/memory.c
> +++ b/xen/common/memory.c
> @@ -1007,6 +1007,26 @@ static long xatp_permission_check(struct domain *d, unsigned int space)
>      return xsm_add_to_physmap(XSM_TARGET, current->domain, d);
>  }
> 
> +/*
> + * Return 0 on any kind of error.  Caller converts to -EINVAL.
> + *
> + * All nonzero values should be repeatable (i.e. derived from some fixed
> + * proerty of the domain), and describe the full resource (i.e. mapping the

s/property/property

> + * result of this call will be the entire resource).

This precludes dynamically adding a resource to a running domain. Do we really want to bake in that restriction?

> + */
> +static unsigned int resource_max_frames(struct domain *d,
> +                                        unsigned int type, unsigned int id)
> +{
> +    switch ( type )
> +    {
> +    case XENMEM_resource_grant_table:
> +        return gnttab_resource_max_frames(d, id);
> +
> +    default:
> +        return arch_resource_max_frames(d, type, id);
> +    }
> +}
> +
>  static int acquire_resource(
>      XEN_GUEST_HANDLE_PARAM(xen_mem_acquire_resource_t) arg)
>  {
> @@ -1018,6 +1038,7 @@ static int acquire_resource(
>       * use-cases then per-CPU arrays or heap allocations may be required.
>       */
>      xen_pfn_t mfn_list[32];
> +    unsigned int max_frames;
>      int rc;
> 
>      if ( copy_from_guest(&xmar, arg, 1) )
> @@ -1026,19 +1047,6 @@ static int acquire_resource(
>      if ( xmar.pad != 0 )
>          return -EINVAL;
> 
> -    if ( guest_handle_is_null(xmar.frame_list) )
> -    {
> -        if ( xmar.nr_frames )
> -            return -EINVAL;
> -
> -        xmar.nr_frames = ARRAY_SIZE(mfn_list);
> -
> -        if ( __copy_field_to_guest(arg, &xmar, nr_frames) )
> -            return -EFAULT;
> -
> -        return 0;
> -    }
> -
>      if ( xmar.nr_frames > ARRAY_SIZE(mfn_list) )
>          return -E2BIG;
> 
> @@ -1050,6 +1058,27 @@ static int acquire_resource(
>      if ( rc )
>          goto out;
> 
> +    max_frames = resource_max_frames(d, xmar.type, xmar.id);
> +
> +    rc = -EINVAL;
> +    if ( !max_frames )
> +        goto out;
> +
> +    if ( guest_handle_is_null(xmar.frame_list) )
> +    {
> +        if ( xmar.nr_frames )
> +            goto out;
> +
> +        xmar.nr_frames = max_frames;
> +
> +        rc = -EFAULT;
> +        if ( __copy_field_to_guest(arg, &xmar, nr_frames) )
> +            goto out;
> +
> +        rc = 0;
> +        goto out;
> +    }
> +
>      switch ( xmar.type )
>      {
>      case XENMEM_resource_grant_table:
> diff --git a/xen/include/asm-x86/mm.h b/xen/include/asm-x86/mm.h
> index 7e74996053..b0caf372a8 100644
> --- a/xen/include/asm-x86/mm.h
> +++ b/xen/include/asm-x86/mm.h
> @@ -649,6 +649,9 @@ static inline bool arch_mfn_in_directmap(unsigned long mfn)
>      return mfn <= (virt_to_mfn(eva - 1) + 1);
>  }
> 
> +unsigned int arch_resource_max_frames(struct domain *d,
> +                                      unsigned int type, unsigned int id);
> +
>  int arch_acquire_resource(struct domain *d, unsigned int type,
>                            unsigned int id, unsigned long frame,
>                            unsigned int nr_frames, xen_pfn_t mfn_list[]);
> diff --git a/xen/include/public/memory.h b/xen/include/public/memory.h
> index 21057ed78e..cea88cf40c 100644
> --- a/xen/include/public/memory.h
> +++ b/xen/include/public/memory.h
> @@ -639,10 +639,18 @@ struct xen_mem_acquire_resource {
>  #define XENMEM_resource_grant_table_id_status 1
> 
>      /*
> -     * IN/OUT - As an IN parameter number of frames of the resource
> -     *          to be mapped. However, if the specified value is 0 and
> -     *          frame_list is NULL then this field will be set to the
> -     *          maximum value supported by the implementation on return.
> +     * IN/OUT
> +     *
> +     * As an IN parameter number of frames of the resource to be mapped.
> +     *
> +     * When frame_list is NULL and nr_frames is 0, this is interpreted as a
> +     * request for the size of the resource, which shall be returned in the
> +     * nr_frames field.
> +     *
> +     * The size of a resource will never be zero, but a nonzero result doesn't
> +     * guarentee that a subsequent mapping request will be successful.  There

s/guarantee/guarantee

  Paul

> +     * are further type/id specific constraints which may change between the
> +     * two calls.
>       */
>      uint32_t nr_frames;
>      uint32_t pad;
> diff --git a/xen/include/xen/grant_table.h b/xen/include/xen/grant_table.h
> index 5a2c75b880..bae4d79623 100644
> --- a/xen/include/xen/grant_table.h
> +++ b/xen/include/xen/grant_table.h
> @@ -57,6 +57,8 @@ int mem_sharing_gref_to_gfn(struct grant_table *gt, grant_ref_t ref,
>  int gnttab_map_frame(struct domain *d, unsigned long idx, gfn_t gfn,
>                       mfn_t *mfn);
> 
> +unsigned int gnttab_resource_max_frames(struct domain *d, unsigned int id);
> +
>  int gnttab_acquire_resource(
>      struct domain *d, unsigned int id, unsigned long frame,
>      unsigned int nr_frames, xen_pfn_t mfn_list[]);
> @@ -93,6 +95,12 @@ static inline int gnttab_map_frame(struct domain *d, unsigned long idx,
>      return -EINVAL;
>  }
> 
> +static inline unsigned int gnttab_resource_max_frames(
> +    struct domain *d, unsigned int id)
> +{
> +    return 0;
> +}
> +
>  static inline int gnttab_acquire_resource(
>      struct domain *d, unsigned int id, unsigned long frame,
>      unsigned int nr_frames, xen_pfn_t mfn_list[])
> diff --git a/xen/include/xen/mm.h b/xen/include/xen/mm.h
> index 1b2c1f6b32..c184dc1db1 100644
> --- a/xen/include/xen/mm.h
> +++ b/xen/include/xen/mm.h
> @@ -686,6 +686,12 @@ static inline void put_page_alloc_ref(struct page_info *page)
>  }
> 
>  #ifndef CONFIG_ARCH_ACQUIRE_RESOURCE
> +static inline unsigned int arch_resource_max_frames(
> +    struct domain *d, unsigned int type, unsigned int id)
> +{
> +    return 0;
> +}
> +
>  static inline int arch_acquire_resource(
>      struct domain *d, unsigned int type, unsigned int id, unsigned long frame,
>      unsigned int nr_frames, xen_pfn_t mfn_list[])
> --
> 2.11.0
>