Re: [PATCH v40 10/24] mm: Add 'mprotect' hook to struct vm_operations_struct

["Haitao Huang" <haitao.huang@linux.intel.com>]

On Mon, 16 Nov 2020 12:00:23 -0600, Dr. Greg <greg@enjellic.com> wrote:

> On Thu, Nov 12, 2020 at 02:41:00PM -0800, Andy Lutomirski wrote:
>
> Good morning, I hope the week is starting well for everyone.
>
>> On Thu, Nov 12, 2020 at 1:31 PM Dave Hansen <dave.hansen@intel.com>  
>> wrote:
>> >
>> > On 11/12/20 12:58 PM, Dr. Greg wrote:
>> > > @@ -270,11 +270,10 @@ static int sgx_vma_mprotect(struct  
>> vm_area_struct *vma,
>> > >                           struct vm_area_struct **pprev, unsigned  
>> long start,
>> > >                           unsigned long end, unsigned long newflags)
>> > >  {
>> > > -     int ret;
>> > > +     struct sgx_encl *encl = vma->vm_private_data;
>> > >
>> > > -     ret = sgx_encl_may_map(vma->vm_private_data, start, end,  
>> newflags);
>> > > -     if (ret)
>> > > -             return ret;
>> > > +     if ( test_bit(SGX_ENCL_INITIALIZED, &encl->flags) )
>> > > +             return -EACCES;
>> > >
>> > >       return mprotect_fixup(vma, pprev, start, end, newflags);
>> > >  }
>> >
>> > This rules out mprotect() on running enclaves.  Does that break any
>> > expectations from enclave authors, or take away capabilities that  
>> folks
>> > need?
>
>> It certainly prevents any scheme in which an enclave coordinates
>> with the outside world to do W-and-then-X JIT inside.  I'm also not
>> convinced it has any real effect at all unless there's some magic I
>> missed to prevent someone from using mmap(2) to effectively change
>> permissions.
>
> The patch that I posted yesterday addresses the security issue for
> both mmap and mprotect by trapping the permission change request at
> the level of the sgx_encl_may_map() function.
>
> With respect to the W-and-then-X JIT issue, the stated purpose of the
> driver is to implement basic SGX functionality, which is SGX1
> semantics, it has been stated formally for a year by the developers
> themselves that they are not entertaining a driver that addresses any
> of the issues associated with non-static memory permissions.
>

The JIT issue is applicable even to SGX1 platforms. We can do EADD with  
EPCM.RWX in sec_info and with PTE.RW, EINIT, then mprotect to set PTE.RX  
when JIT is done.

Haitao