linux-kernel - Re: [PATCH v2] x86/xen/efi: Initialize UEFI secure boot state during dom0 boot

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Message-ID: <1522770281.4522.14.camel@HansenPartnership.com>
Date:   Tue, 03 Apr 2018 08:44:41 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Daniel Kiper <daniel.kiper@...cle.com>, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        xen-devel@...ts.xenproject.org
Cc:     ard.biesheuvel@...aro.org, boris.ostrovsky@...cle.com,
        eric.snowberg@...cle.com, hpa@...or.com, jgross@...e.com,
        konrad.wilk@...cle.com, mingo@...hat.com, tglx@...utronix.de
Subject: Re: [PATCH v2] x86/xen/efi: Initialize UEFI secure boot state
 during dom0 boot

On Tue, 2018-04-03 at 16:39 +0200, Daniel Kiper wrote:
> Initialize UEFI secure boot state during dom0 boot. Otherwise the
> kernel
> may not even know that it runs on secure boot enabled platform.
> 
> Signed-off-by: Daniel Kiper <daniel.kiper@...cle.com>
> ---
>  arch/x86/xen/efi.c                        |   57
> +++++++++++++++++++++++++++++
>  drivers/firmware/efi/libstub/secureboot.c |    3 ++
>  2 files changed, 60 insertions(+)
> 
> diff --git a/arch/x86/xen/efi.c b/arch/x86/xen/efi.c
> index a18703b..1804b27 100644
> --- a/arch/x86/xen/efi.c
> +++ b/arch/x86/xen/efi.c
> @@ -115,6 +115,61 @@ static efi_system_table_t __init
> *xen_efi_probe(void)
>  	return &efi_systab_xen;
>  }
>  
> +/*
> + * Determine whether we're in secure boot mode.
> + *
> + * Please keep the logic in sync with
> + * drivers/firmware/efi/libstub/secureboot.c:efi_get_secureboot().
> + */
> +static enum efi_secureboot_mode xen_efi_get_secureboot(void)
> +{
> +	static efi_guid_t efi_variable_guid =
> EFI_GLOBAL_VARIABLE_GUID;
> +	static efi_guid_t shim_guid = EFI_SHIM_LOCK_GUID;
> +	efi_status_t status;
> +	u8 moksbstate, secboot, setupmode;
> +	unsigned long size;
> +
> +	size = sizeof(secboot);
> +	status = efi.get_variable(L"SecureBoot", &efi_variable_guid,
> +				  NULL, &size, &secboot);
> +
> +	if (status == EFI_NOT_FOUND)
> +		return efi_secureboot_mode_disabled;
> +
> +	if (status != EFI_SUCCESS)
> +		goto out_efi_err;
> +
> +	size = sizeof(setupmode);
> +	status = efi.get_variable(L"SetupMode", &efi_variable_guid,
> +				  NULL, &size, &setupmode);
> +
> +	if (status != EFI_SUCCESS)
> +		goto out_efi_err;
> +
> +	if (secboot == 0 || setupmode == 1)
> +		return efi_secureboot_mode_disabled;
> +
> +	/* See if a user has put the shim into insecure mode. */
> +	size = sizeof(moksbstate);
> +	status = efi.get_variable(L"MokSBStateRT", &shim_guid,
> +				  NULL, &size, &moksbstate);
> +
> +	/* If it fails, we don't care why. Default to secure. */
> +	if (status != EFI_SUCCESS)
> +		goto secure_boot_enabled;
> +
> +	if (moksbstate == 1)
> +		return efi_secureboot_mode_disabled;
> +
> + secure_boot_enabled:
> +	pr_info("UEFI Secure Boot is enabled.\n");
> +	return efi_secureboot_mode_enabled;
> +
> + out_efi_err:
> +	pr_err("Could not determine UEFI Secure Boot status.\n");
> +	return efi_secureboot_mode_unknown;
> +}
> +

This looks like a bad idea: you're duplicating the secure boot check in

drivers/firmware/efi/libstub/secureboot.c

Which is an implementation of policy.  If we have to have policy in the
kernel, it should really only be in one place to prevent drift; why
can't you simply use the libstub efi_get_secureboot() so we're not
duplicating the implementation of policy?

James