linux-kernel - Re: [PATCH v17 18/23] platform/x86: Intel SGX driver

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Date:   Mon, 17 Dec 2018 10:36:13 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...el.com>, x86@...nel.org,
        platform-driver-x86@...r.kernel.org, linux-sgx@...r.kernel.org,
        nhorman@...hat.com, npmccallum@...hat.com, serge.ayoun@...el.com,
        shay.katz-zamir@...el.com, haitao.huang@...ux.intel.com,
        andriy.shevchenko@...ux.intel.com, tglx@...utronix.de,
        kai.svahn@...el.com, mark.shanahan@...el.com, luto@...capital.net,
        Suresh Siddha <suresh.b.siddha@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v17 18/23] platform/x86: Intel SGX driver

On Mon, Dec 17, 2018 at 08:01:02PM +0200, Jarkko Sakkinen wrote:
> On Mon, Dec 17, 2018 at 09:45:40AM -0800, Dave Hansen wrote:
> > > +struct sgx_encl *sgx_encl_alloc(struct sgx_secs *secs)
> > > +{
> > ...
> > > +	kref_init(&encl->refcount);
> > > +	INIT_LIST_HEAD(&encl->add_page_reqs);
> > > +	INIT_RADIX_TREE(&encl->page_tree, GFP_KERNEL);
> > > +	mutex_init(&encl->lock);
> > > +	INIT_WORK(&encl->add_page_work, sgx_add_page_worker);
> > > +
> > > +	encl->mm = current->mm;  <---------------------------------> +	encl->base = secs->base;
> > > +	encl->size = secs->size;
> > > +	encl->ssaframesize = secs->ssa_frame_size;
> > > +	encl->backing = backing;
> > > +
> > > +	return encl;
> > > +}
> > 
> > How is this OK without taking a reference on the mm?

It's subtle and the ordering is all kinds of weird, but technically we
are taking a reference on mm when the mmu_notifier is registered in
sgx_encl_create().  sgx_encl_alloc() and sgx_encl_create() are always
called in tandem and with mm->mm_users > 0, so we'll never use encl->mm
without holding a reference to mm.  We need to comment the weirdness
or maybe register the notifier before

> > I have a feeling a bunch of your bugs with the mmu notifiers and so
> > forth are because the refcounting is wrong here.

Eh, not really.  Maybe the mmu_notifier is more subtle, e.g. calling
do_unmap() after mmput() would be quite obvious, but there's no
fundamental bug, we just haven't needed to touch VMAs during release
prior to moving away from shmem.

> > Sean's SGX_ENCL_MM_RELEASED would, I think be unnecessary if you just
> > take a refcount here and release it when the enclave is destroyed.
> 
> Right, atomic_inc(encl->mm->count) here and once when releasing.
> 
> The we would not even need the whole mmu notifier in the first place.

I'm pretty sure doing mmget() would result in circular dependencies and
a zombie enclave.  In the do_exit() case where a task is abruptly killed:
 
  - __mmput() is never called because the enclave holds a ref
  - sgx_encl_release() is never be called because its VMAs hold refs
  - sgx_vma_close() is never called because __mmput()->exit_mmap() is
    blocked and the process itself is dead, i.e. won't unmap anything.