linux-kernel - Re: [PATCH v4 03/12] x86/mtrr: support setting MTRR state for software defined MTRRs

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Date:   Mon, 20 Mar 2023 20:05:43 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v4 03/12] x86/mtrr: support setting MTRR state for
 software defined MTRRs

On Mon, Mar 06, 2023 at 05:34:16PM +0100, Juergen Gross wrote:
> +/**
> + * mtrr_overwrite_state - set static MTRR state
> + *
> + * Used to set MTRR state via different means (e.g. with data obtained from
> + * a hypervisor).
> + * Is allowed only for special cases when running virtualized. Must be called
> + * from the x86_init.hyper.init_platform() hook. X86_FEATURE_MTRR must be off.
> + */
> +void mtrr_overwrite_state(struct mtrr_var_range *var, unsigned int num_var,
> +			  mtrr_type def_type)
> +{
> +	unsigned int i;
> +
> +	if (WARN_ON(mtrr_state_set ||
> +		    hypervisor_is_type(X86_HYPER_NATIVE) ||

Why that check?

> +		    !cpu_feature_enabled(X86_FEATURE_HYPERVISOR) ||
> +		    (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP) &&
> +		     !hv_is_isolation_supported() &&
> +		     !cpu_feature_enabled(X86_FEATURE_XENPV) &&
> +		     !cpu_feature_enabled(X86_FEATURE_TDX_GUEST))))

This is unparseable. Please split it into separate checks:

	if (WARN_ON(mtrr_state_set))
		return;

	if (WARN_ON(!cpu_feature_enabled(X86_FEATURE_HYPERVISOR)))
		return;

	...

and add comments above each one why we're testing this.


> +		return;
> +
> +	/* Disable MTRR in order to disable MTRR modifications. */
> +	setup_clear_cpu_cap(X86_FEATURE_MTRR);
> +
> +	if (var) {
> +		if (num_var > MTRR_MAX_VAR_RANGES) {
> +			pr_warn("Trying to overwrite MTRR state with %u variable entries\n",
> +				num_var);
> +			num_var = MTRR_MAX_VAR_RANGES;
> +		}
> +		for (i = 0; i < num_var; i++)
> +			mtrr_state.var_ranges[i] = var[i];
> +		num_var_ranges = num_var;
> +	}
> +
> +	mtrr_state.def_type = def_type;
> +	mtrr_state.enabled |= MTRR_STATE_MTRR_ENABLED;
> +
> +	mtrr_state_set = 1;
> +}
> +
>  /**
>   * mtrr_type_lookup - look up memory type in MTRR
>   *
> diff --git a/arch/x86/kernel/cpu/mtrr/mtrr.c b/arch/x86/kernel/cpu/mtrr/mtrr.c
> index 7596ebeab929..5fe62ee0361b 100644
> --- a/arch/x86/kernel/cpu/mtrr/mtrr.c
> +++ b/arch/x86/kernel/cpu/mtrr/mtrr.c
> @@ -666,6 +666,15 @@ void __init mtrr_bp_init(void)
>  	const char *why = "(not available)";
>  	unsigned int phys_addr;
>  
> +	if (mtrr_state.enabled) {

I'm guessing the proper detection of that weird state should be:

	/*
	 * Check for the software overwrite of MTRR state, only for generic case.
	 * See mtrr_overwrite_state().
	 */
	if (!cpu_feature_enabled(X86_FEATURE_MTRR) &&
	    mtrr_state.enabled) {
		...


> +		/* Software overwrite of MTRR state, only for generic case. */
> +		mtrr_calc_physbits(true);
> +		init_table();
> +		pr_info("MTRRs set to read-only\n");
> +
> +		return;
> +	}
> +

-- 
Regards/Gruss,
    Boris.

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