Re: [PATCH v2] x86/vmx: Add command line option to enable EPT without PAT

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 16/04/2014 23:52, Aravindh Puthiyaparambil wrote:
> The fix for XSA-60 disables EPT if PAT is not available. This patch
> adds a command line option called "ept_without_pat", that allows EPT to
> be enabled even when PAT is not present. This is to enable Xen to run as
> a nested guest with EPT on hypervisors that have nested EPT but not
> nested PAT.
>
> Signed-off-by: Aravindh Puthiyaparambil <aravindp@cisco.com>
> Cc: Jun Nakajima <jun.nakajima@intel.com>
> Cc: Eddie Dong <eddie.dong@intel.com>
> Cc: Kevin Tian <kevin.tian@intel.com>

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

>
> ---
> Changes from version 1:
> 1. Fix and update documentation with suggestion from Andrew Cooper.
> 2. Remove redundant assignment.
>
>  docs/misc/xen-command-line.markdown | 14 ++++++++++++++
>  xen/arch/x86/hvm/vmx/vmx.c          |  5 ++++-
>  2 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
> index 87de2dc..138fee9 100644
> --- a/docs/misc/xen-command-line.markdown
> +++ b/docs/misc/xen-command-line.markdown
> @@ -523,6 +523,20 @@ Either force retrieval of monitor EDID information via VESA DDC, or
>  disable it (edid=no). This option should not normally be required
>  except for debugging purposes.
>  
> +### ept\_without\_pat (Intel)
> +> `= <boolean>`
> +
> +> Default: `false`
> +
> +Allow EPT to be enabled when PAT is not present. 
> +
> +*Warning:*
> +Due to CVE-2013-2212, PAT is by default required as a prerequisite for
> +using EPT.  If you are not using PCI Passthrough, or trust the guest
> +administrator who would be using passthrough, then the PAT requirement
> +can be relaxed.  This option is useful for nested virtualisation cases
> +where the outer hypervisor does not expose PAT functionality to Xen.
> +
>  ### extra\_guest\_irqs
>  > `= [<domU number>][,<dom0 number>]`
>  
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> index 180cf6c..fee81c9 100644
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -58,6 +58,9 @@
>  #include <asm/hvm/nestedhvm.h>
>  #include <asm/event.h>
>  
> +static bool_t __initdata opt_ept_without_pat;
> +boolean_param("ept_without_pat", opt_ept_without_pat);
> +
>  enum handler_return { HNDL_done, HNDL_unhandled, HNDL_exception_raised };
>  
>  static void vmx_ctxt_switch_from(struct vcpu *v);
> @@ -1724,7 +1727,7 @@ const struct hvm_function_table * __init start_vmx(void)
>       * Do not enable EPT when (!cpu_has_vmx_pat), to prevent security hole
>       * (refer to http://xenbits.xen.org/xsa/advisory-60.html).
>       */
> -    if ( cpu_has_vmx_ept && cpu_has_vmx_pat )
> +    if ( cpu_has_vmx_ept && (cpu_has_vmx_pat || opt_ept_without_pat) )
>      {
>          vmx_function_table.hap_supported = 1;
>