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Date:   Fri, 17 May 2019 10:55:00 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Stephen Smalley <sds@tycho.nsa.gov>,
        "Xing, Cedric" <cedric.xing@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        LSM List <linux-security-module@vger.kernel.org>,
        Paul Moore <paul@paul-moore.com>,
        Eric Paris <eparis@parisplace.org>,
        "selinux@vger.kernel.org" <selinux@vger.kernel.org>,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        Jethro Beekman <jethro@fortanix.com>,
        "Hansen, Dave" <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Dr. Greg" <greg@enjellic.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "linux-sgx@vger.kernel.org" <linux-sgx@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "nhorman@redhat.com" <nhorman@redhat.com>,
        "npmccallum@redhat.com" <npmccallum@redhat.com>,
        "Ayoun, Serge" <serge.ayoun@intel.com>,
        "Katz-zamir, Shay" <shay.katz-zamir@intel.com>,
        "Huang, Haitao" <haitao.huang@intel.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        "Svahn, Kai" <kai.svahn@intel.com>, Borislav Petkov <bp@alien8.de>,
        Josh Triplett <josh@joshtriplett.org>,
        "Huang, Kai" <kai.huang@intel.com>,
        David Rientjes <rientjes@google.com>
Subject: Re: SGX vs LSM (Re: [PATCH v20 00/28] Intel SGX1 support)
Message-ID: <20190517175500.GE15006@linux.intel.com>
References: <CALCETrX2ovRx3Rre+1_xC-q6CiybyLjQ-gmB4FZF_qCZ-Qd+4A@mail.gmail.com>
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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@intel.com> 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;
        }

	...
}