linux-kernel - Re: [PATCH v14 09/19] x86/mm: x86/sgx: Signal SEGV_SGXERR for #PFs w/ PF

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Date:   Thu, 27 Sep 2018 08:53:27 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Platform Driver <platform-driver-x86@...r.kernel.org>,
        nhorman@...hat.com, npmccallum@...hat.com,
        "Ayoun, Serge" <serge.ayoun@...el.com>, shay.katz-zamir@...el.com,
        linux-sgx@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v14 09/19] x86/mm: x86/sgx: Signal SEGV_SGXERR for #PFs w/
 PF_SGX

On 09/27/2018 08:39 AM, Jarkko Sakkinen wrote:
> On Thu, Sep 27, 2018 at 07:58:41AM -0700, Dave Hansen wrote:
>> On 09/27/2018 06:42 AM, Jarkko Sakkinen wrote:
>>>> 	This flag is 1 if the exception is unrelated to paging and
>>>> 	resulted from violation of SGX-specific access-control
>>>> 	requirements. ... such a violation can occur only if there
>>>> 	is no ordinary page fault...
>>>>
>>>> This is pretty important.  It means that *none* of the other
>>>> paging-related stuff that we're doing applies.
>>>>
>>>> We also need to clarify how this can happen.  Is it through something
>>>> than an app does, or is it solely when the hardware does something under
>>>> the covers, like suspend/resume.
>>> When you change page permissions lets say with mprotect after the and
>>> try to do an invalid access according to the EPCM permissions this can
>>> happen.
>>
>> So, there are pages that are non-executable, non-readable, or
>> non-writable both via the page tables and via underlying SGX
>> permissions.  Then, we allow an mprotect() and a later access will
>> result in one of these SGX faults?
> 
> The permissions are intersection of PTE and EPCM permissions.

Right, but this *fault* bit is not.

> EPCM permissions are part of the enclave measurement. For SGX1 they are
> static. For SGX2 they can be changed with EMODPR/EACCEPT protocol (i.e.
> measurement can be updated after enclave initialization).

What does this all have to do with enclave measurement?

>> What permissions are these, exactly?  Is it even a good idea to let that
>> mprotect() go through in the first place?
> 
> You define RWX for each page when you do EADD.

Are those permissions reflected into the VMAs mapping the enclave memory?