linux-kernel - Re: [RFC PATCH 8/9] LSM: x86/sgx: Introduce ->enclave

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Date:   Mon, 3 Jun 2019 11:45:03 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Stephen Smalley <sds@...ho.nsa.gov>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Cedric Xing <cedric.xing@...el.com>,
        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>,
        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 8/9] LSM: x86/sgx: Introduce ->enclave_load() hook for
 Intel SGX

...
>>> 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.