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

[Sean Christopherson]

On Fri, May 17, 2019 at 10:43:01AM -0700, Andy Lutomirski wrote:
> 
> > On May 17, 2019, at 10:29 AM, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
> > 
> > AIUI, having FILE__WRITE and FILE__EXECUTE on /dev/sgx/enclave would allow
> > *any* enclave/process to map EPC as RWX.  Moving to anon inodes and thus
> > PROCESS__EXECMEM achieves per-process granularity.
> 
> How does anon_inode make any difference?  Anon_inode is not the same thing as
> anon_vma.

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.

static int file_map_prot_check(struct file *file, unsigned long prot, int shared)
{
        const struct cred *cred = current_cred();
        u32 sid = cred_sid(cred);
        int rc = 0;

        if (default_noexec &&
            (prot & PROT_EXEC) && (!file || IS_PRIVATE(file_inode(file)) ||
                                   (!shared && (prot & PROT_WRITE)))) {
                /*
                 * We are making executable an anonymous mapping or a
                 * private file mapping that will also be writable.
                 * This has an additional check.
                 */
                rc = avc_has_perm(&selinux_state,
                                  sid, sid, SECCLASS_PROCESS,
                                  PROCESS__EXECMEM, NULL);
                if (rc)
                        goto error;
        }

	...
}