Re: [Xen-devel] [edk2-devel] [PATCH v4 20/35] OvmfPkg/XenPlatformPei: Introduce XenPvhDetected

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

On Mon, Jul 29, 2019 at 04:39:29PM +0100, Anthony PERARD wrote:
> XenPvhDetected() can be used to figure out if OVMF has started via the
> Xen PVH entry point.
> 
> Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1689
> Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> Acked-by: Laszlo Ersek <lersek@xxxxxxxxxx>

Thanks, I've got a comment, but it can be fixed afterwards if required.

> ---
>  OvmfPkg/XenPlatformPei/Platform.h |  5 +++++
>  OvmfPkg/XenPlatformPei/Xen.c      | 13 +++++++++++++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/OvmfPkg/XenPlatformPei/Platform.h 
> b/OvmfPkg/XenPlatformPei/Platform.h
> index 4a80057bdc..db9a62572f 100644
> --- a/OvmfPkg/XenPlatformPei/Platform.h
> +++ b/OvmfPkg/XenPlatformPei/Platform.h
> @@ -99,6 +99,11 @@ XenHvmloaderDetected (
>    VOID
>    );
>  
> +BOOLEAN
> +XenPvhDetected (
> +  VOID
> +  );
> +
>  VOID
>  AmdSevInitialize (
>    VOID
> diff --git a/OvmfPkg/XenPlatformPei/Xen.c b/OvmfPkg/XenPlatformPei/Xen.c
> index 29b42b746c..71fe5de446 100644
> --- a/OvmfPkg/XenPlatformPei/Xen.c
> +++ b/OvmfPkg/XenPlatformPei/Xen.c
> @@ -214,6 +214,19 @@ XenHvmloaderDetected (
>    return (mXenHvmloaderInfo != NULL);
>  }
>  
> +BOOLEAN
> +XenPvhDetected (
> +  VOID
> +  )
> +{
> +  //
> +  // This function should only be used after XenConnect
> +  //
> +  ASSERT (mXenInfo.VersionMajor != 0);

That's IMO dangerous. Using the version as an indication that
XenConnect has run seems like a bad idea, since returning a major
version of 0 is a valid number to return. Can't you check against
something else that doesn't depends on hypervisor provided data? (ie:
like some allocations or such that happen in XenConnect)

A paranoid could provider could even return major == 0 and minor == 0
in order to attempt to hide the Xen version used, since guests are not
supposed to infer anything from the Xen version, available hypervisor
features are reported by other means.

Thanks, Roger.

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