linux-kernel - Re: [RFC PATCH v2 2/5] x86/sgx: Require userspace to define enclave pages' protection bits

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Message-ID: <CALCETrWv9FYDtiHMfnfH==jE00tt7F22t-zcnP+XjfRCQgLr7A@mail.gmail.com>
Date:   Mon, 10 Jun 2019 12:15:25 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Xing, Cedric" <cedric.xing@...el.com>
Cc:     "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        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 v2 2/5] x86/sgx: Require userspace to define enclave
 pages' protection bits

On Mon, Jun 10, 2019 at 11:29 AM Xing, Cedric <cedric.xing@...el.com> wrote:
>
> > From: Christopherson, Sean J
> > Sent: Wednesday, June 05, 2019 7:12 PM
> >
> > +/**
> > + * sgx_map_allowed - check vma protections against the associated
> > enclave page
> > + * @encl:    an enclave
> > + * @start:   start address of the mapping (inclusive)
> > + * @end:     end address of the mapping (exclusive)
> > + * @prot:    protection bits of the mapping
> > + *
> > + * Verify a userspace mapping to an enclave page would not violate the
> > +security
> > + * requirements of the *kernel*.  Note, this is in no way related to
> > +the
> > + * page protections enforced by hardware via the EPCM.  The EPCM
> > +protections
> > + * can be directly extended by the enclave, i.e. cannot be relied upon
> > +by the
> > + * kernel for security guarantees of any kind.
> > + *
> > + * Return:
> > + *   0 on success,
> > + *   -EACCES if the mapping is disallowed
> > + */
> > +int sgx_map_allowed(struct sgx_encl *encl, unsigned long start,
> > +                 unsigned long end, unsigned long prot) {
> > +     struct sgx_encl_page *page;
> > +     unsigned long addr;
> > +
> > +     prot &= (VM_READ | VM_WRITE | VM_EXEC);
> > +     if (!prot || !encl)
> > +             return 0;
> > +
> > +     mutex_lock(&encl->lock);
> > +
> > +     for (addr = start; addr < end; addr += PAGE_SIZE) {
> > +             page = radix_tree_lookup(&encl->page_tree, addr >>
> > PAGE_SHIFT);
> > +
> > +             /*
> > +              * Do not allow R|W|X to a non-existent page, or protections
> > +              * beyond those of the existing enclave page.
> > +              */
> > +             if (!page || (prot & ~page->prot))
> > +                     return -EACCES;
>
> In SGX2, pages will be "mapped" before being populated.
>
> Here's a brief summary for those who don't have enough background on how new EPC pages could be added to a running enclave in SGX2:
>   - There are 2 new instructions - EACCEPT and EAUG.
>   - EAUG is used by SGX module to add (augment) a new page to an existing enclave. The newly added page is *inaccessible* until the enclave *accepts* it.
>   - EACCEPT is the instruction for an enclave to accept a new page.
>
> And the s/w flow for an enclave to request new EPC pages is expected to be something like the following:
>   - The enclave issues EACCEPT at the linear address that it would like a new page.
>   - EACCEPT results in #PF, as there's no page at the linear address above.
>   - SGX module is notified about the #PF, in form of its vma->vm_ops->fault() being called by kernel.
>   - SGX module EAUGs a new EPC page at the fault address, and resumes the enclave.
>   - EACCEPT is reattempted, and succeeds at this time.

This seems like an odd workflow.  Shouldn't the #PF return back to
untrusted userspace so that the untrusted user code can make its own
decision as to whether it wants to EAUG a page there as opposed to,
say, killing the enclave or waiting to keep resource usage under
control?