Re: [PATCH v38 13/24] x86/sgx: Add SGX_IOC_ENCLAVE_ADD_PAGES

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Fri, Sep 18, 2020 at 05:09:19PM -0700, Sean Christopherson wrote:
> On Fri, Sep 18, 2020 at 03:39:32PM +0300, Jarkko Sakkinen wrote:
> > On Fri, Sep 18, 2020 at 03:20:39PM +0300, Jarkko Sakkinen wrote:
> > > On Thu, Sep 17, 2020 at 07:09:40PM -0700, Sean Christopherson wrote:
> > > > On Thu, Sep 17, 2020 at 01:35:10PM -0500, Haitao Huang wrote:
> > > > > On Thu, 17 Sep 2020 11:02:06 -0500, Jarkko Sakkinen
> > > > > <jarkko.sakkinen@linux.intel.com> wrote:
> > > > > > 
> > > > > > Right, I do get the OOM case but wouldn't in that case the reasonable
> > > > > > thing to do destroy the enclave that is not even running? I mean that
> > > > > > means that we are globally out of EPC.
> > > > > > 
> > > > > 
> > > > > I would say it could be a policy, but not the only one. If it does not make
> > > > > much difference to kernel, IMHO we should  not set it in stone now.
> > > > > Debugging is also huge benefit to me.
> > > > 
> > > > Agreed, an EPC cgroup is the proper way to define/enforce what happens when
> > > > there is EPC pressure.  E.g. if process A is consuming 99% of the EPC, then
> > > > it doesn't make sense to unconditionally kill enclaves from process B.  If
> > > > the admin wants to give process A priority, so be it, but such a decision
> > > > shouldn't be baked into the kernel.
> > > > 
> > > > This series obviously doesn't provide an EPC cgroup, but that doesn't mean
> > > > we can't make decisions that will play nice with a cgroup in the future.
> > > 
> > > Here's the core issue why the API "as is used to be" does not work:
> > > 
> > > 	if (ret == -EIO) {
> > > 		mutex_lock(&encl->lock);
> > > 		sgx_encl_destroy(encl);
> > > 		mutex_unlock(&encl->lock);
> > > 	}
> > > 
> > > It would be better to instead whitelist *when* the enclave is preserved.
> > > 
> > > 	if (ret != -ENOMEM) {
> > > 		mutex_lock(&encl->lock);
> > > 		sgx_encl_destroy(encl);
> > > 		mutex_unlock(&encl->lock);
> > > 	}
> > > 
> > > That is the information we *deterministically* want to know. Otherwise,
> > > we will live in ultimate chaos.
> > > 
> > > Only this way can caller know when there are means to continue, and when
> > > to quit. I.e. the code is whitelisting wrong way around currently.
> > 
> > Actually since the state cannot corrupt unless EADD or EEXTEND fails it
> > is fine to have the enclave alive on any other error condition. I think
> 
> EADD and EEXTEND failure don't corrupt state.  Killing the enclave if EEXTEND
> fails makes sense because failure at that point is either due to a kernel bug
> or loss of EPC, both of which are fatal to the enclave.

This is also true. I meant by corrupt state e.g. a kernel bug, which
causes uninitalizes pages go the free queue.

I'd rephrase this in kdoc as: "The function deinitializes enclave and
returns -EIO when EPC is lost, while entering to a new power cycle".

Documentation describes only legit behaviour, let's ignore the corrupt
part.

> EADD is a little different, e.g. it could fault due to a bad source address,
> in which case the failure is not technically fatal.  But, Jarkko wanted to
> have consistent behavior for EADD and EEXTEND failures, and practically
> speaking the enclave is probably hosed anyways if EADD fails, i.e. killing the
> enclave on EADD failure isn't a sticking point (for me).

We need to figure out own return value for EADD, but I agree with this.

I would go with -EFAULT as we do when source VMA is no available. Does
this make sense to you?

/Jarkko