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Subject: Re: [PATCH v38 10/24] mm: Add vm_ops->mprotect()
To:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Haitao Huang <haitao.huang@linux.intel.com>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        linux-sgx@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        Jethro Beekman <jethro@fortanix.com>,
        Darren Kenny <darren.kenny@oracle.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        asapek@google.com, Borislav Petkov <bp@alien8.de>,
        "Xing, Cedric" <cedric.xing@intel.com>, chenalexchen@google.com,
        Conrad Parker <conradparker@google.com>, cyhanish@google.com,
        "Huang, Haitao" <haitao.huang@intel.com>,
        Josh Triplett <josh@joshtriplett.org>,
        "Huang, Kai" <kai.huang@intel.com>,
        "Svahn, Kai" <kai.svahn@intel.com>, Keith Moyer <kmoy@google.com>,
        Christian Ludloff <ludloff@google.com>,
        Neil Horman <nhorman@redhat.com>,
        Nathaniel McCallum <npmccallum@redhat.com>,
        Patrick Uiterwijk <puiterwijk@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Thomas Gleixner <tglx@linutronix.de>, yaozhangx@google.com
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Message-ID: <c680f7bd-2d82-6477-707f-cd03aae4b4aa@intel.com>
Date:   Thu, 24 Sep 2020 12:39:24 -0700
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On 9/24/20 12:28 PM, Sean Christopherson wrote:
> On Thu, Sep 24, 2020 at 02:11:37PM -0500, Haitao Huang wrote:
>> On Wed, 23 Sep 2020 08:50:56 -0500, Jarkko Sakkinen
>> <jarkko.sakkinen@linux.intel.com> wrote:
>>> I'll categorically deny noexec in the next patch set version.
>>>
>>> /Jarkko
>> There are use cases supported currently in which enclave binary is received
>> via IPC/RPC and held in buffers before EADD. Denying noexec altogether would
>> break those, right?
> No.  noexec only applies to file-backed VMAs, what you're describing is loading
> an enclave from an anon VMA, which will still have VM_MAYEXEC.

Maybe I'm just stupid, but I still don't get the scenario that's being
thwarted or why it is valuable.  The SDM is worthless on what EMODPE
does or what its restrictions are.

In pseudo-C, it's something logically like this for the "nice" case:

	ptr = mmap("/some/executable", PROT_EXEC);
	ioctl(sgx_fd, ADD_ENCLAVE_PAGE, SGX_PROT_EXEC, ptr, size);
	mmap(sgx_fd);
	EENTER;

And we're trying to thwart:

	ptr = mmap("/mnt/noexec/file", PROT_READ);
	ioctl(sgx_fd, ADD_ENCLAVE_PAGE, SGX_PROT_EXEC, ptr, size);
	mmap(sgx_fd);
	EENTER;

because that loads data into the enclave which is executable but which
was not executable normally.  But, we're allowing this from anonymous
memory, so this would seem to work:

	ptr = mmap("/mnt/noexec/file", PROT_READ);
	buffer = malloc(PAGE_SIZE);
	memcpy(buffer, ptr, PAGE_SIZE);
	// need mprotect(buf, PROT_EXEC)???
	ioctl(sgx_fd, ADD_ENCLAVE_PAGE, SGX_PROT_EXEC, buffer, size);
	mmap(sgx_fd);
	EENTER;

and give the same result.  What am I missing?