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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 17 Dec 2018 11:25:47 -0800
Message-ID: <CALCETrUCAvsaO71422gbaJtuZTbx8Y7pUm7yF3zQkktOvRedzw@mail.gmail.com>
Subject: Re: [PATCH v17 18/23] platform/x86: Intel SGX driver
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        X86 ML <x86@kernel.org>,
        Platform Driver <platform-driver-x86@vger.kernel.org>,
        linux-sgx@vger.kernel.org, nhorman@redhat.com,
        npmccallum@redhat.com, "Ayoun, Serge" <serge.ayoun@intel.com>,
        shay.katz-zamir@intel.com,
        Haitao Huang <haitao.huang@linux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Svahn, Kai" <kai.svahn@intel.com>, mark.shanahan@intel.com,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Darren Hart <dvhart@infradead.org>,
        Andy Shevchenko <andy@infradead.org>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
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On Mon, Dec 17, 2018 at 11:17 AM Dave Hansen <dave.hansen@intel.com> wrote:
>
> On 12/17/18 11:12 AM, Andy Lutomirski wrote:
> > So I'm not saying that you shouldn't do it the way you are now, but I
> > do think that the changelog or at least some emails should explain
> > *why* the enclave needs to keep a pointer to the creating process's
> > mm.  And, if you do keep the current model, it would be nice to
> > understand what happens if you do something awful like mremap()ing an
> > enclave, or calling madvise on it, or otherwise abusing the vma.  Or
> > doing fork(), for that matter.
>
> Yeah, the code is built to have one VMA and only one VMA per enclave.
> You need to go over the origin of this restriction and what enforces this.

There is a sad historical reason that you may regret keeping this
restriction.  There are plenty of pieces of code out there that think
it's reasonable to spawn a subprocess by calling fork() and then
execve().  (This is *not* a sensible thing to do.  One should use
posix_spawn() or some CLONE_VM variant.  But even fairly recent
posix_spawn() implementations will fork().  So the driver has to do
*something* sensible on fork() or a bunch of things that use SGX
unsuspectingly via, for example, PKCS #11, are going to be very sad.
I suppose you could make enclaves just not show up in the fork()ed
children, but then you have a different problem: creating an enclave
and then doing daemon() won't work.

Yes, POSIX traditions are rather silly.