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

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Message-ID: <CALCETrVe0jhAWAFmx+NFEjJcijSJv2LDVC7cUXi0w99kNKjh_g@mail.gmail.com>
Date:   Tue, 4 Jun 2019 13:16:04 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        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 2/9] x86/sgx: Do not naturally align MAP_FIXED address

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.