Re: [PATCH v20 00/28] Intel SGX1 support

[Sean Christopherson]

On Wed, May 15, 2019 at 01:35:31PM +0300, Jarkko Sakkinen wrote:
> On Tue, May 14, 2019 at 01:45:27PM -0700, Sean Christopherson wrote:
> > On Tue, May 14, 2019 at 08:13:36AM -0700, Andy Lutomirski wrote:
> > > I think it's as simple as requiring that, if SECINFO.X is set, then
> > > the src pointer points to the appropriate number of bytes of
> > > executable memory.  (Unless there's some way for an enclave to change
> > > SECINFO after the fact -- is there?)
> > 
> > Nit: SECINFO is just the struct passed to EADD, I think what you're really
> > asking is "can the EPCM permissions be changed after the fact".
> > 
> > And the answer is, yes.
> > 
> > On SGX2 hardware, the enclave can extend the EPCM permissions at runtime
> > via ENCLU[EMODPE], e.g. to make a page writable.
> 
> Small correction: it is EMODPR.

No, I'm referring to EMODPE, note the ENCLU classification.

> Anyway, it is good to mention that these would require EACCEPT from the
> enclave side. In order to take advantage of this is in a malicous
> enclave, one would require SELinux/IMA/whatnot policy to have permitted
> it in the first place.

EMODPE doesn't require EACCEPT or any equivalent from the kernel.  As
you alluded to, the page tables would still need to allow PROT_EXEC.  I
was simply trying to answer Andy's question regarding SECINFO.

> Thus, it cannot be said that it breaks the security policy if this would
> happen because policy has allowed to use the particular enclave.