linux-kernel - RE: [RFC PATCH 5/9] x86/sgx: Restrict mapping without an enclave page to PROT

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Date:   Mon, 3 Jun 2019 06:28:36 +0000
From:   "Xing, Cedric" <cedric.xing@...el.com>
To:     "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        "Jarkko Sakkinen" <jarkko.sakkinen@...ux.intel.com>
CC:     Andy Lutomirski <luto@...nel.org>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Paul Moore <paul@...l-moore.com>,
        Eric Paris <eparis@...isplace.org>,
        "selinux@...r.kernel.org" <selinux@...r.kernel.org>,
        Jethro Beekman <jethro@...tanix.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "nhorman@...hat.com" <nhorman@...hat.com>,
        "npmccallum@...hat.com" <npmccallum@...hat.com>,
        "Ayoun, Serge" <serge.ayoun@...el.com>,
        "Katz-zamir, Shay" <shay.katz-zamir@...el.com>,
        "Huang, Haitao" <haitao.huang@...el.com>,
        "Andy Shevchenko" <andriy.shevchenko@...ux.intel.com>,
        "Svahn, Kai" <kai.svahn@...el.com>, Borislav Petkov <bp@...en8.de>,
        Josh Triplett <josh@...htriplett.org>,
        "Huang, Kai" <kai.huang@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        "Roberts, William C" <william.c.roberts@...el.com>,
        "Tricca, Philip B" <philip.b.tricca@...el.com>
Subject: RE: [RFC PATCH 5/9] x86/sgx: Restrict mapping without an enclave
 page to PROT_NONE

> From: Christopherson, Sean J
> Sent: Friday, May 31, 2019 4:32 PM
> 
> To support LSM integration, SGX will require userspace to explicitly specify the allowed
> protections for each page.  The allowed protections will be supplied to and modified by
> LSMs (based on their policies).
> To prevent userspace from circumventing the allowed protections, do not allow
> PROT_{READ,WRITE,EXEC} mappings to an enclave without an associated enclave page (which
> will track the allowed protections).

This is unnecessary. 

For mprotect(), LSM shall validate @prot against existing pages with applicable global flags (e.g. FILE__EXECMOD/PROCESS__EXECMEM in the case of SELinux). 

For mmap(), SGX driver could invoke security_file_mprotect() explicitly to have LSM validate requested protection.

In the case where there's no page associated with an VMA, security_file_mprotect() shall still dictate whether to allow/deny the request. LSM internally is able to track existence/nonexistence of enclave pages. If there's no page, there's no conflict so the decision shall only depend on global flags (if any). Afterwards, #PF may trigger SGX driver to EAUG, in which case security_enclave_load() will be invoked and LSM could decide whether to approve/decline EAUG request.