linux-kernel - Re: SGX vs LSM (Re: [PATCH v20 00/28] Intel SGX1 support)

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Date:   Fri, 17 May 2019 11:21:24 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        "Xing, Cedric" <cedric.xing@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Paul Moore <paul@...l-moore.com>,
        Eric Paris <eparis@...isplace.org>,
        "selinux@...r.kernel.org" <selinux@...r.kernel.org>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Jethro Beekman <jethro@...tanix.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Dr. Greg" <greg@...ellic.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "nhorman@...hat.com" <nhorman@...hat.com>,
        "npmccallum@...hat.com" <npmccallum@...hat.com>,
        "Ayoun, Serge" <serge.ayoun@...el.com>,
        "Katz-zamir, Shay" <shay.katz-zamir@...el.com>,
        "Huang, Haitao" <haitao.huang@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Svahn, Kai" <kai.svahn@...el.com>, Borislav Petkov <bp@...en8.de>,
        Josh Triplett <josh@...htriplett.org>,
        "Huang, Kai" <kai.huang@...el.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: SGX vs LSM (Re: [PATCH v20 00/28] Intel SGX1 support)

On Fri, May 17, 2019 at 11:04:22AM -0700, Linus Torvalds wrote:
> On Fri, May 17, 2019 at 10:55 AM Sean Christopherson
> <sean.j.christopherson@...el.com> wrote:
> >
> > In this snippet, IS_PRIVATE() is true for anon inodes, false for
> > /dev/sgx/enclave.  Because EPC memory is always shared, SELinux will never
> > check PROCESS__EXECMEM for mprotect() on/dev/sgx/enclave.
> 
> Why _does_ the memory have to be shared? Shared mmap() is
> fundamentally less secure than private mmap, since by definition it
> means "oh, somebody else has access to it too and might modify it
> under us".
> 
> Why does the SGX logic care about things like that? Normal executables
> are just private mappings of an underlying file, I'm not sure why the
> SGX interface has to have that shared thing, and why the interface has
> to have a device node in the first place when  you have system calls
> for setup anyway.
> 
> So why don't the system calls just work on perfectly normal anonymous
> mmap's? Why a device node, and why must it be shared to begin with?

I agree that conceptually EPC is private memory, but because EPC is
managed as a separate memory pool, SGX tags it VM_PFNMAP and manually
inserts PFNs, i.e. EPC effectively it gets classified as IO memory. 

And vmf_insert_pfn_prot() doesn't like writable private IO mappings:

   BUG_ON((vma->vm_flags & VM_PFNMAP) && is_cow_mapping(vma->vm_flags));