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

[Haitao Huang]

On Mon, 16 Nov 2020 12:00:23 -0600, Dr. Greg <greg@xxxxxxxxxxxx> 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@xxxxxxxxx>  
> > 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