linux-kernel - RE: [RFC PATCH 2/9] x86/sgx: Do not naturally align MAP

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Message-ID: <960B34DE67B9E140824F1DCDEC400C0F654EDBDE@ORSMSX116.amr.corp.intel.com>
Date:   Tue, 4 Jun 2019 22:10:22 +0000
From:   "Xing, Cedric" <cedric.xing@...el.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
CC:     "Christopherson, Sean J" <sean.j.christopherson@...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" <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 2/9] x86/sgx: Do not naturally align MAP_FIXED
 address

> From: linux-sgx-owner@...r.kernel.org [mailto:linux-sgx-
> owner@...r.kernel.org] On Behalf Of Andy Lutomirski
> Sent: Tuesday, June 04, 2019 1:16 PM
> 
> On Tue, Jun 4, 2019 at 4:50 AM Jarkko Sakkinen
> <jarkko.sakkinen@...ux.intel.com> wrote:
> >
> > On Fri, May 31, 2019 at 04:31:52PM -0700, Sean Christopherson wrote:
> > > SGX enclaves have an associated Enclave Linear Range (ELRANGE) that
> > > is tracked and enforced by the CPU using a base+mask approach,
> > > similar to how hardware range registers such as the variable MTRRs.
> > > As a result, the ELRANGE must be naturally sized and aligned.
> > >
> > > To reduce boilerplate code that would be needed in every userspace
> > > enclave loader, the SGX driver naturally aligns the mmap() address
> > > and also requires the range to be naturally sized.  Unfortunately,
> > > SGX fails to grant a waiver to the MAP_FIXED case, e.g. incorrectly
> > > rejects mmap() if userspace is attempting to map a small slice of an
> existing enclave.
> > >
> > > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> >
> > Why you want to allow mmap() to be called multiple times? mmap() could
> > be allowed only once with PROT_NONE and denied afterwards. Is this for
> > sending fd to another process that would map already existing enclave?
> >
> > I don't see any checks for whether the is enclave underneath. Also, I
> > think that in all cases mmap() callback should allow only PROT_NONE as
> > permissions for clarity even if it could called multiple times.
> >
> 
> What's the advantage to only allowing PROT_NONE?  The idea here is to
> allow a PROT_NONE map followed by some replacemets that overlay it for
> the individual segments.  Admittedly, mprotect() can do the same thing,
> but disallowing mmap() seems at least a bit surprising.

Disallowing mmap() is not only surprising but also unnecessary.

A bit off topic here. This mmap()/mprotect() discussion reminds me a question (guess for Jarkko): Now that vma->vm_file->private_data keeps a pointer to the enclave, why do we store it again in vma->vm_private? It isn't a big deal but non-NULL vm_private does prevent mprotect() from merging adjacent VMAs.