Re: [PATCH v10 05/16] x86/virt/tdx: Add skeleton to enable TDX on demand

[Dave Hansen]

On 3/13/23 18:50, Huang, Kai wrote:
> On Mon, 2023-03-13 at 16:49 -0700, Isaku Yamahata wrote:
>> On Sun, Mar 12, 2023 at 11:08:44PM +0000,
>> "Huang, Kai" <kai.huang@xxxxxxxxx> wrote:
>>
>>> On Wed, 2023-03-08 at 14:27 -0800, Isaku Yamahata wrote:
>>>>> +
>>>>> +static int try_init_module_global(void)
>>>>> +{
>>>>> +       int ret;
>>>>> +
>>>>> +       /*
>>>>> +        * The TDX module global initialization only needs to be done
>>>>> +        * once on any cpu.
>>>>> +        */
>>>>> +       spin_lock(&tdx_global_init_lock);
>>>>> +
>>>>> +       if (tdx_global_init_status & TDX_GLOBAL_INIT_DONE) {
>>>>> +               ret = tdx_global_init_status & TDX_GLOBAL_INIT_FAILED ?
>>>>> +                       -EINVAL : 0;
>>>>> +               goto out;
>>>>> +       }
>>>>> +
>>>>> +       /* All '0's are just unused parameters. */
>>>>> +       ret = seamcall(TDH_SYS_INIT, 0, 0, 0, 0, NULL, NULL);
>>>>> +
>>>>> +       tdx_global_init_status = TDX_GLOBAL_INIT_DONE;
>>>>> +       if (ret)
>>>>> +               tdx_global_init_status |= TDX_GLOBAL_INIT_FAILED;
>>>>
>>>> If entropy is lacking (rdrand failure), TDH_SYS_INIT can return TDX_SYS_BUSY.
>>>> In such case, we should allow the caller to retry or make this function retry
>>>> instead of marking error stickily.
>>>
>>> The spec says:
>>>
>>> TDX_SYS_BUSY        The operation was invoked when another TDX module
>>>             operation was in progress. The operation may be retried.
>>>
>>> So I don't see how entropy is lacking is related to this error.  Perhaps you
>>> were mixing up with KEY.CONFIG?
>>
>> TDH.SYS.INIT() initializes global canary value.  TDX module is compiled with
>> strong stack protector enabled by clang and canary value needs to be
>> initialized.  By default, the canary value is stored at
>> %fsbase:<STACK_CANARY_OFFSET 0x28>
>>
>> Although this is a job for libc or language runtime, TDX modules has to do it
>> itself because it's stand alone.
>>
>> From tdh_sys_init.c
>> _STATIC_INLINE_ api_error_type tdx_init_stack_canary(void)
>> {
>>     ia32_rflags_t rflags = {.raw = 0};
>>     uint64_t canary;
>>     if (!ia32_rdrand(&rflags, &canary))
>>     {
>>         return TDX_SYS_BUSY;
>>     }
>> ...
>>     last_page_ptr->stack_canary.canary = canary;
>>
>>
> 
> Then it is a hidden behaviour of the TDX module that is not reflected in the
> spec.

This is true.  Could you please go ask the TDX module folks to fix this up?

> I am not sure whether we should handle because:
> 
> 1) This is an extremely rare case.  Kernel would be basically under attack if
> such error happened.  In the current series we don't handle such case in
> KEY.CONFIG either but just leave a comment (see patch 13).

Rare, yes.  Under attack?  I'm not sure where you get that from.  Look
at the SDM:

> Under heavy load, with multiple cores executing RDRAND in parallel, it is possible, though unlikely, for the demand
> of random numbers by software processes/threads to exceed the rate at which the random number generator
> hardware can supply them. This will lead to the RDRAND instruction returning no data transitorily. The RDRAND
> instruction indicates the occurrence of this rare situation by clearing the CF flag.

That doesn't talk about attacks.

> 2) Not sure whether this will be changed in the future.
> 
> So I think we should keep as is.

TDX_SYS_BUSY really is missing some nuance.  You *REALLY* want to retry
RDRAND failures.  But, if you have VMM locking and don't expect two
users calling into the TDX module then TDX_SYS_BUSY from a busy *module*
is a bad (and probably fatal) signal.

I suspect we should just throw a few retries in the seamcall()
infrastructure to retry in the case of TDX_SYS_BUSY.  It'll take care of
RDRAND failures.  If a retry loop fails to resolve it, then we should
probably dump a warning and return an error.

Just do this once, in common code.