linux-kernel - RE: [PATCH v2 2/3] hyperv: Change hv_root

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Message-ID:
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Date: Thu, 20 Feb 2025 19:17:30 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Nuno Das Neves <nunodasneves@...ux.microsoft.com>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
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Subject: RE: [PATCH v2 2/3] hyperv: Change hv_root_partition into a function

From: Nuno Das Neves <nunodasneves@...ux.microsoft.com> Sent: Thursday, February 20, 2025 10:33 AM
> 
> Introduce hv_current_partition_type to store the partition type
> as an enum.
> 
> Right now this is limited to guest or root partition, but there will
> be other kinds in future and the enum is easily extensible.
> 
> Set up hv_current_partition_type early in Hyper-V initialization with
> hv_identify_partition_type(). hv_root_partition() just queries this
> value, and shouldn't be called before that.
> 
> Making this check into a function sets the stage for adding a config
> option to gate the compilation of root partition code. In particular,
> hv_root_partition() can be stubbed out always be false if root
> partition support isn't desired.
> 
>

[snip]
 
> +void hv_identify_partition_type(void)
> +{
> +	/* Assume guest role */
> +	hv_current_partition_type = HV_PARTITION_TYPE_GUEST;
> +	/*
> +	 * Check partition creation and cpu management privileges
> +	 *
> +	 * Hyper-V should never specify running as root and as a Confidential
> +	 * VM. But to protect against a compromised/malicious Hyper-V trying
> +	 * to exploit root behavior to expose Confidential VM memory, ignore
> +	 * the root partition setting if also a Confidential VM.
> +	 */
> +	if ((ms_hyperv.priv_high & HV_CREATE_PARTITIONS) &&
> +	    (ms_hyperv.priv_high & HV_CPU_MANAGEMENT) &&
> +	    !(ms_hyperv.priv_high & HV_ISOLATION)) {
> +		pr_info("Hyper-V: running as root partition\n");
> +		if (IS_ENABLED(CONFIG_MSHV_ROOT))

I'll have to rescind the "Reviewed-by:" that I just gave. There's a patch
sequencing problem in that CONFIG_MSHV_ROOT doesn't exist yet.
It's added in Patch 3 of the series.  Because it doesn't exist, the
IS_ENABLED() will always return 'false', which isn't fatal in the sense
of causing a compile error.  But the code won't run in the root partition
because hv_current_partition_type isn't set.

Michael

> +			hv_current_partition_type = HV_PARTITION_TYPE_ROOT;
> +		else
> +			pr_crit("Hyper-V: CONFIG_MSHV_ROOT not enabled!\n");
> +	}
> +}