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

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Message-Id: <3FECC02D-C65C-4D35-B538-D32EC7D722D5@amacapital.net>
Date:   Fri, 17 May 2019 11:53:00 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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 May 17, 2019, at 11:21 AM, Sean Christopherson <sean.j.christopherson@...el.com> wrote:
> 
>> 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));

I don’t see how it could be anonymous even in principle.  The kernel can’t *read* the memory — how could we possibly CoW it?  And we can’t share an RO backing pages between two different enclaves because the CPU won’t let us — each EPC page belongs to a particular enclave.  And fork()ing an enclave is right out.

So I agree that MAP_ANONYMOUS would be nice conceptually, but I don’t see how it would work.