linux-kernel - Re: [PATCH v38 10/24] mm: Add vm

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Date:   Tue, 22 Sep 2020 08:11:14 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        linux-sgx@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jethro Beekman <jethro@...tanix.com>,
        Darren Kenny <darren.kenny@...cle.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        asapek@...gle.com, Borislav Petkov <bp@...en8.de>,
        "Xing, Cedric" <cedric.xing@...el.com>, chenalexchen@...gle.com,
        Conrad Parker <conradparker@...gle.com>, cyhanish@...gle.com,
        "Huang, Haitao" <haitao.huang@...el.com>,
        Josh Triplett <josh@...htriplett.org>,
        "Huang, Kai" <kai.huang@...el.com>,
        "Svahn, Kai" <kai.svahn@...el.com>, Keith Moyer <kmoy@...gle.com>,
        Christian Ludloff <ludloff@...gle.com>,
        Neil Horman <nhorman@...hat.com>,
        Nathaniel McCallum <npmccallum@...hat.com>,
        Patrick Uiterwijk <puiterwijk@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>, yaozhangx@...gle.com
Subject: Re: [PATCH v38 10/24] mm: Add vm_ops->mprotect()

On 9/22/20 5:58 AM, Jarkko Sakkinen wrote:
> Intel Sofware Guard eXtensions (SGX) allows creation of executable blobs
> called enclaves, of which page permissions are defined when the enclave

"of which" => "for which"

> is first loaded. Once an enclave is loaded and initialized, it can be
> mapped to the process address space.

Could you compare and contrast this a *bit* with existing executables?
What's special about SGX?  ELF executables have page permissions inside
the binary too.  Why don't we use this mechanism for them?

> Enclave permissions can be dynamically modified by using ENCLS[EMODPE]

I'm not sure this sentence matters.  I'm not sure why I care what the
instruction is named that does this.  But, it _sounds_ here like an
enclave can adjust its own permissions directly with ENCLS[EMODPE].

> instruction. We want to limit its use to not allow higher permissions than
> the ones defined when the enclave was first created.

Rather than higher and lower, please use stronger and weaker.

Also, please get rid of the "we".

> Add 'mprotect' hook to vm_ops, so that we can implement a callback for SGX
> that will check that {mmap, mprotect}() permissions do not surpass any of
> the page permissions in the address range defined.

"check" => "ensure"

> This is required in order to be able to make any access control decisions
> when enclave pages are loaded.
Now I'm confused.  I actually don't think I have a strong understanding
of how an enclave actually gets loaded, how mmap() and mprotect() are
normally used and what this hook is intended to thwart.