linux-kernel - Re: [RFC PATCH 7/9] x86/sgx: Enforce noexec filesystem restriction for enclaves

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Date:   Wed, 5 Jun 2019 18:01:34 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Cedric Xing <cedric.xing@...el.com>,
        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,
        Jethro Beekman <jethro@...tanix.com>,
        Dave Hansen <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,
        Andrew Morton <akpm@...ux-foundation.org>, nhorman@...hat.com,
        npmccallum@...hat.com, Serge Ayoun <serge.ayoun@...el.com>,
        Shay Katz-zamir <shay.katz-zamir@...el.com>,
        Haitao Huang <haitao.huang@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Kai Svahn <kai.svahn@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Josh Triplett <josh@...htriplett.org>,
        Kai Huang <kai.huang@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        William Roberts <william.c.roberts@...el.com>,
        Philip Tricca <philip.b.tricca@...el.com>
Subject: Re: [RFC PATCH 7/9] x86/sgx: Enforce noexec filesystem restriction
 for enclaves

On Wed, Jun 05, 2019 at 06:10:18PM +0300, Jarkko Sakkinen wrote:
> On Tue, Jun 04, 2019 at 01:25:10PM -0700, Andy Lutomirski wrote:
> > On Tue, Jun 4, 2019 at 9:26 AM Jarkko Sakkinen
> > <jarkko.sakkinen@...ux.intel.com> wrote:
> > >
> > > On Fri, May 31, 2019 at 04:31:57PM -0700, Sean Christopherson wrote:
> > > > Do not allow an enclave page to be mapped with PROT_EXEC if the source
> > > > page is backed by a file on a noexec file system.
> > > >
> > > > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> > >
> > > Why don't you just check in sgx_encl_add_page() that whether the path
> > > comes from noexec and deny if SECINFO contains X?
> > >
> > 
> > SECINFO seems almost entirely useless for this kind of thing because
> > of SGX2.  I'm thinking that SECINFO should be completely ignored for
> > anything other than its required architectural purpose.
> 
> Not exactly sure why using it to pass the RWX bits to EADD ioctl would
> cause anything to SGX2 support.

Andy was pointing out that with SGX2 the enclave can do ENCLU[EMODPE] to
make the page executable, e.g. add the page with SECINFO.R and then
mprotect() the enclave VMA (whose vm_file == /dev/sgx/enclave) PROT_EXEC.
We could hard enforce SECINFO, i.e. set the enclave page's protection bits
directly from SECINFO, but that would neuter SGX2, e.g. would break
converting RW to RX.