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Subject: Re: [RFC PATCH 8/9] LSM: x86/sgx: Introduce ->enclave_load() hook for
 Intel SGX
To:     Stephen Smalley <sds@tycho.nsa.gov>,
        Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Cedric Xing <cedric.xing@intel.com>,
        James Morris <jmorris@namei.org>,
        "Serge E . Hallyn" <serge@hallyn.com>,
        LSM List <linux-security-module@vger.kernel.org>,
        Paul Moore <paul@paul-moore.com>,
        Eric Paris <eparis@parisplace.org>, selinux@vger.kernel.org,
        Jethro Beekman <jethro@fortanix.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        linux-sgx@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>, nhorman@redhat.com,
        npmccallum@redhat.com, Serge Ayoun <serge.ayoun@intel.com>,
        Shay Katz-zamir <shay.katz-zamir@intel.com>,
        Haitao Huang <haitao.huang@intel.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Kai Svahn <kai.svahn@intel.com>,
        Borislav Petkov <bp@alien8.de>,
        Josh Triplett <josh@joshtriplett.org>,
        Kai Huang <kai.huang@intel.com>,
        David Rientjes <rientjes@google.com>,
        William Roberts <william.c.roberts@intel.com>,
        Philip Tricca <philip.b.tricca@intel.com>
References: <20190531233159.30992-1-sean.j.christopherson@intel.com>
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 <d9ef6314-de91-ddcb-3d18-8155cd37e7be@tycho.nsa.gov>
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From:   Dave Hansen <dave.hansen@intel.com>
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Date:   Mon, 3 Jun 2019 11:45:03 -0700
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...
>>> What ensures that the mapping referenced by src can't be changed
>>> to an entirely different one (with a different vm_file) between
>>> the time of check (here) and the time of use?
>> 
>> Nothing.  Holding mmap_sem across copy_from_user() would suffice, 
>> correct?
> 
> I don't believe you can do that; copy_from_user() could stall 
> indefinitely.  Not sure how to do what you want here or if it requires
> changing the interface.

Holding mmap_sem for *read* is OK since you can handle page faults
underneath it.  Holding it for write is not.

But, holding it for read also locks out the writers which might be
messing with vm_file or other parts of the VMA.

Holding it for read for a long time is OK.  It's obviously not ideal,
but it is something we do widely today.