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

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Date:   Fri, 14 Jun 2019 18:18:28 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     "Xing, Cedric" <cedric.xing@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        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 2/9] x86/sgx: Do not naturally align MAP_FIXED address

On Thu, Jun 13, 2019 at 10:14:51AM -0700, Sean Christopherson wrote:
> > 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? 
> 
> mmap_sem is only held when reclaim is touching PTEs, e.g. to test/clear
> its accessed bit and to zap the PTE.  The liveliness of the enclave needs
> to be guaranteed for the entire duration of reclaim, e.g. we can't have
> the enclave disappearing when we go to do EWB.  It's also worth nothing
> that a single reclaim may operate on more than one mmap_sem, as enclaves
> can be shared across processes (mm_structs).

Anyway, the takeaway I got from this is that encl->refcount does not
need to be updated for VMAs (sent a patch to linux-sgx that I plan
merge).

/Jarkko