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

[Sean Christopherson]

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 at intel.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));