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

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Message-ID: <20181217193322.GA29785@linux.intel.com>
Date:   Mon, 17 Dec 2018 21:33:22 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@...el.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 10:48:58AM -0800, Sean Christopherson wrote:
> 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.

Noticed that too. mmu_notifier_unregister() requires a valid mm.

/Jarkko