linux-kernel - Re: [PATCH v19 038/130] KVM: TDX: create/destroy VM structure

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Date: Tue, 16 Apr 2024 09:40:41 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>
Cc: isaku.yamahata@...el.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
	Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com,
	Sean Christopherson <seanjc@...gle.com>,
	Sagi Shahar <sagis@...gle.com>, Kai Huang <kai.huang@...el.com>,
	chen.bo@...el.com, hang.yuan@...el.com, tina.zhang@...el.com,
	Sean Christopherson <sean.j.christopherson@...el.com>,
	isaku.yamahata@...ux.intel.com
Subject: Re: [PATCH v19 038/130] KVM: TDX: create/destroy VM structure

On Mon, Apr 15, 2024 at 04:17:35PM +0800,
Xiaoyao Li <xiaoyao.li@...el.com> wrote:

> On 2/26/2024 4:25 PM, isaku.yamahata@...el.com wrote:
> 
> ...
> 
> > +
> > +	kvm_tdx->tdcs_pa = tdcs_pa;
> > +	for (i = 0; i < tdx_info->nr_tdcs_pages; i++) {
> > +		err = tdh_mng_addcx(kvm_tdx->tdr_pa, tdcs_pa[i]);
> > +		if (err == TDX_RND_NO_ENTROPY) {
> > +			/* Here it's hard to allow userspace to retry. */
> > +			ret = -EBUSY;
> 
> So userspace is expected to stop creating TD and quit on this?
> 
> If so, it exposes an DOS attack surface that malicious users in another can
> drain the entropy with busy-loop on RDSEED.
> 
> Can you clarify why it's hard to allow userspace to retry? To me, it's OK to
> retry that "teardown" cleans everything up, and userspace and issue the
> KVM_TDX_INIT_VM again.

The current patch has complicated error recovery path.  After simplifying
the code, it would be possible to return -EAGAIN in this patch.

For the retry case, we need to avoid TDH.MNG.CREATE() and TDH.MNG.KEY.CONFIG().
-- 
Isaku Yamahata <isaku.yamahata@...el.com>