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

[Sathyanarayanan Kuppuswamy]

On 11/21/22 1:44 AM, Huang, Kai wrote:
> On Sun, 2022-11-20 at 19:51 -0800, Sathyanarayanan Kuppuswamy wrote:
>>
>> 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?
> 
> It was quoted from below upstream commit id (I only kept the one that I quoted
> to save space):

Ok.

> 
> commit b8d1d163604bd1e600b062fb00de5dc42baa355f (tag: x86_apic_for_v6.1_rc1,
> tip/x86/apic)
> Author: Daniel Sneddon <daniel.sneddon@xxxxxxxxxxxxxxx>
> Date:   Tue Aug 16 16:19:42 2022 -0700
> 
>     x86/apic: Don't disable x2APIC if locked
>     
>     ....
>     
>     The MMIO/xAPIC interface has some problems, most notably the APIC LEAK
>     [1].  This bug allows an attacker to use the APIC MMIO interface to
>     extract data from the SGX enclave.
>     
>     ....
>     
>     [1]: https://aepicleak.com/aepicleak.pdf
>     
>     Signed-off-by: Daniel Sneddon <daniel.sneddon@xxxxxxxxxxxxxxx>
>     Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>     Acked-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>     Tested-by: Neelima Krishnan <neelima.krishnan@xxxxxxxxx>
>     Link:
> https://lkml.kernel.org/r/20220816231943.1152579-1-daniel.sneddon@xxxxxxxxxxxxxxx
> 
> 
>>
>>> [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@xxxxxxxxx/
>>> Link: https://lore.kernel.org/lkml/ba80b303-31bf-d44a-b05d-5c0f83038798@xxxxxxxxx/
>>> Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
>>> ---
>>>
>>> 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.
> 
> Sorry which "double space"?

Before Just disable. It looked like double space. Is it not?

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

Since it is a failure case, I thought pr_warn would be better. It is up
to you.

> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer