Re: [PATCH v39 13/24] x86/sgx: Add SGX_IOC_ENCLAVE_ADD_PAGES

[Jarkko Sakkinen <jarkko@kernel.org>]

On Sun, Oct 18, 2020 at 08:03:11AM +0300, Jarkko Sakkinen wrote:
> > > +	mmap_read_lock(current->mm);
> > > +	mutex_lock(&encl->lock);
> > > +
> > > +	/*
> > > +	 * Insert prior to EADD in case of OOM.
> > 
> > I wouldn't say OOM.  Maybe:
> > 
> > 	xa_insert() and EADD can both fail.  But xa_insert() is easier
> > 	to unwind so do it first.
> > 
> > >                                              EADD modifies MRENCLAVE, i.e.
> > 
> > What is MRENCLAVE?
> 
> The measurement stored in SECS. I'm wondering  with xarray, is it
> possible to preallocate entry without inserting anything?
> 
> Then we could get rid of this unwind and also would not need to
> take encl->lock in sgx_encl_may_map().

I'm still a bit confused with the unfamiliar Xarray API but I think I
got it:

1. xa_insert() with a NULL entry reserves index and more importantly
   does the memory allocation.
2. xa_cmpxchg() with the enclave page, if EADD and EEXTEND's succceed.
3. xa_release() otherwise.

This way sgx_encl_may_map() will never see a stale enclave page when it
does the permission check, even if encl->lock is not taken.

I mean right now I have to take both xas lock and enclave lock, which
is terrible but this will take care of it.

I will rewrite the comment to something more reasonable, once I've done
this code change.

The reason for doing insert first is that, if we get -ENOMEM after
successful EADD and EEXTEND's we have a legit microarchitectural state
but you cannot rollback a hash (MRENCLAVE), so game is anyway over
because your data structures are not in sync.

If -ENOMEM comes before, everything is still in sync and we don't have
invalidate the enclave.

/Jarkko