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

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Date:   Mon, 13 Mar 2023 21:02:00 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "david@...hat.com" <david@...hat.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Luck, Tony" <tony.luck@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        "Brown, Len" <len.brown@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v10 05/16] x86/virt/tdx: Add skeleton to enable TDX on
 demand

On Tue, Mar 14, 2023 at 01:50:40AM +0000,
"Huang, Kai" <kai.huang@...el.com> 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@...el.com> 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.  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).
> 
> 2) Not sure whether this will be changed in the future.
> 
> So I think we should keep as is.

TDX 1.5 spec introduced TDX_RND_NO_ENTROPY status code.  For TDX 1.0, let's
postpone it to TDX 1.5 activity.
-- 
Isaku Yamahata <isaku.yamahata@...il.com>