Re: [Xen-devel] [PATCH] common/gnttab: Introduce command line feature controls

[Juergen Gross <jgross@xxxxxxxx>]

On 24/08/17 16:50, Andrew Cooper wrote:
> This patch was originally a workaround for XSA-226.  Since then, transitive
> grants are believed to be functioning properly, and the defaults have changed
> appropriately.
> 
> However, for those people who chose to use the workaround (especially from an
> attack surface mitigation point of view), retain the ability for the host
> administrator to choose.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
> CC: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: Tim Deegan <tim@xxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> ---
>  docs/misc/xen-command-line.markdown | 13 +++++++++++
>  xen/common/grant_table.c            | 44 
> +++++++++++++++++++++++++++++++++++--
>  2 files changed, 55 insertions(+), 2 deletions(-)
> 
> diff --git a/docs/misc/xen-command-line.markdown 
> b/docs/misc/xen-command-line.markdown
> index 4002eab..78c7b51 100644
> --- a/docs/misc/xen-command-line.markdown
> +++ b/docs/misc/xen-command-line.markdown
> @@ -868,6 +868,19 @@ Controls EPT related features.
>  
>  Specify which console gdbstub should use. See **console**.
>  
> +### gnttab
> +> `= List of [ max_ver:<integer>, transitive ]`
> +
> +> Default: `gnttab=max_ver:2,transitive`
> +
> +Control various aspects of the grant table behaviour available to guests.
> +
> +* `max_ver` Select the maximum grant table version to offer to guests.  Valid
> +version are 1 and 2.
> +* `transitive` Permit or disallow the use of transitive grants.  Note that 
> the
> +use of grant table v2 without transitive grants is an ABI breakage from the
> +guests point of view.

So shouldn't there be a way for the guest to query the support of
transient grants?

> +
>  ### gnttab\_max\_frames
>  > `= <integer>`
>  
> diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
> index 36895aa..f9c313d 100644
> --- a/xen/common/grant_table.c
> +++ b/xen/common/grant_table.c
> @@ -50,6 +50,42 @@ integer_param("gnttab_max_nr_frames", max_nr_grant_frames);
>  unsigned int __read_mostly max_grant_frames;
>  integer_param("gnttab_max_frames", max_grant_frames);
>  
> +static unsigned int __read_mostly opt_gnttab_max_version = 2;
> +static bool __read_mostly opt_transitive_grants = true;
> +
> +static void __init parse_gnttab(char *s)

Do you mind using:

static int __init parse_gnttab(const char *s)

in order to comply with my runtime parameter series?


Juergen

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