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

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Date:   Thu, 13 Jun 2019 16:47:06 +0000
From:   "Xing, Cedric" <cedric.xing@...el.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>
CC:     Andy Lutomirski <luto@...nel.org>,
        "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: Jarkko Sakkinen [mailto:jarkko.sakkinen@...ux.intel.com]
> Sent: Thursday, June 13, 2019 6:48 AM
> 
> On Thu, Jun 06, 2019 at 06:37:10PM +0300, Jarkko Sakkinen wrote:
> > On Wed, Jun 05, 2019 at 01:14:04PM -0700, Andy Lutomirski wrote:
> > >
> > >
> > > > On Jun 5, 2019, at 8:17 AM, Jarkko Sakkinen
> <jarkko.sakkinen@...ux.intel.com> wrote:
> > > >
> > > >> On Tue, Jun 04, 2019 at 10:10:22PM +0000, Xing, Cedric wrote:
> > > >> 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.
> > > >
> > > > Same semantics as with a regular mmap i.e. you can close the file
> > > > and still use the mapping.
> > > >
> > > >
> > >
> > > The file should be properly refcounted — vm_file should not go away
> while it’s mapped.
> 
> mm already takes care of that so vm_file does not go away. Still we need
> an internal refcount for enclaves to synchronize with the swapper. In
> summary nothing needs to be done.

I don't get this. The swapper takes a read lock on mm->mmap_sem, which locks the vma, which in turn reference counts vma->vm_file. Why is the internal refcount still needed? 

> 
> > Right, makes sense. It is easy one to change essentially just removing
> > internal refcount from sgx_encl and using file for the same. I'll
> > update this to my tree along with the changes to remove LKM/ACPI bits
> ASAP.
> 
> /Jarkko