linux-kernel - Re: [PATCH v7 03/20] x86/virt/tdx: Disable TDX if X2APIC is not enabled

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Date:   Sun, 20 Nov 2022 19:51:31 -0800
From:   Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     linux-mm@...ck.org, seanjc@...gle.com, pbonzini@...hat.com,
        dave.hansen@...el.com, dan.j.williams@...el.com,
        rafael.j.wysocki@...el.com, kirill.shutemov@...ux.intel.com,
        ying.huang@...el.com, reinette.chatre@...el.com,
        len.brown@...el.com, tony.luck@...el.com, peterz@...radead.org,
        ak@...ux.intel.com, isaku.yamahata@...el.com, chao.gao@...el.com,
        bagasdotme@...il.com, sagis@...gle.com, imammedo@...hat.com
Subject: Re: [PATCH v7 03/20] x86/virt/tdx: Disable TDX if X2APIC is not
 enabled



On 11/20/22 4:26 PM, Kai Huang wrote:
> The MMIO/xAPIC interface has some problems, most notably the APIC LEAK

"some problems" looks more generic. May be we can be specific here. Like
it has security issues?

> [1].  This bug allows an attacker to use the APIC MMIO interface to
> extract data from the SGX enclave.
> 
> TDX is not immune from this either.  Early check X2APIC and disable TDX
> if X2APIC is not enabled, and make INTEL_TDX_HOST depend on X86_X2APIC.
> 
> [1]: https://aepicleak.com/aepicleak.pdf
> 
> Link: https://lore.kernel.org/lkml/d6ffb489-7024-ff74-bd2f-d1e06573bb82@intel.com/
> Link: https://lore.kernel.org/lkml/ba80b303-31bf-d44a-b05d-5c0f83038798@intel.com/
> Signed-off-by: Kai Huang <kai.huang@...el.com>
> ---
> 
> v6 -> v7:
>  - Changed to use "Link" for the two lore links to get rid of checkpatch
>    warning.
> 
> ---
>  arch/x86/Kconfig            |  1 +
>  arch/x86/virt/vmx/tdx/tdx.c | 11 +++++++++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index cced4ef3bfb2..dd333b46fafb 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1958,6 +1958,7 @@ config INTEL_TDX_HOST
>  	depends on CPU_SUP_INTEL
>  	depends on X86_64
>  	depends on KVM_INTEL
> +	depends on X86_X2APIC
>  	help
>  	  Intel Trust Domain Extensions (TDX) protects guest VMs from malicious
>  	  host and certain physical attacks.  This option enables necessary TDX
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> index 982d9c453b6b..8d943bdc8335 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.c
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -12,6 +12,7 @@
>  #include <linux/printk.h>
>  #include <asm/msr-index.h>
>  #include <asm/msr.h>
> +#include <asm/apic.h>
>  #include <asm/tdx.h>
>  #include "tdx.h"
>  
> @@ -81,6 +82,16 @@ static int __init tdx_init(void)
>  		goto no_tdx;
>  	}
>  
> +	/*
> +	 * TDX requires X2APIC being enabled to prevent potential data
> +	 * leak via APIC MMIO registers.  Just disable TDX if not using
> +	 * X2APIC.

Remove the double space.

> +	 */
> +	if (!x2apic_enabled()) {
> +		pr_info("Disable TDX as X2APIC is not enabled.\n");

pr_warn()?

> +		goto no_tdx;
> +	}
> +
>  	return 0;
>  no_tdx:
>  	clear_tdx();

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer