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

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Date:   Mon, 17 Dec 2018 10:48:58 -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 08:43:33PM +0200, Jarkko Sakkinen wrote:
> On Mon, Dec 17, 2018 at 10:36:13AM -0800, Sean Christopherson wrote:
> > 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.
> 
> Right, it does, you are absolutely right. Tried it and removed the
> commit already.
> 
> Well, what we came up from your suggestion i.e. setting mm to NULL
> and checking that is very subtle change and does not have any such
> circular dependencies. We'll go with that.

We can't set mm to NULL as we need it to unregister the notifier, and
I'm fairly certain attempting to unregister in the release callback
will deadlock.