linux-kernel - Re: RFC: userspace exception fixups

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Message-ID: <ce304dba-0b02-02e2-4de8-212cd3bd1876@fortanix.com>
Date:   Mon, 19 Nov 2018 05:17:26 +0000
From:   Jethro Beekman <jethro@...tanix.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>
CC:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Florian Weimer <fweimer@...hat.com>,
        Linux API <linux-api@...r.kernel.org>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        X86 ML <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rich Felker <dalias@...c.org>,
        "nhorman@...hat.com" <nhorman@...hat.com>,
        "npmccallum@...hat.com" <npmccallum@...hat.com>,
        "Ayoun, Serge" <serge.ayoun@...el.com>,
        "shay.katz-zamir@...el.com" <shay.katz-zamir@...el.com>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: RFC: userspace exception fixups

On 2018-11-18 18:32, Jarkko Sakkinen wrote:
> On Sun, Nov 18, 2018 at 09:15:48AM +0200, Jarkko Sakkinen wrote:
>> On Thu, Nov 01, 2018 at 10:53:40AM -0700, Andy Lutomirski wrote:
>>> Hi all-
>>>
>>> The people working on SGX enablement are grappling with a somewhat
>>> annoying issue: the x86 EENTER instruction is used from user code and
>>> can, as part of its normal-ish operation, raise an exception.  It is
>>> also highly likely to be used from a library, and signal handling in
>>> libraries is unpleasant at best.
>>>
>>> There's been some discussion of adding a vDSO entry point to wrap
>>> EENTER and do something sensible with the exceptions, but I'm
>>> wondering if a more general mechanism would be helpful.
>>
>> I haven't really followed all of this discussion because I've been busy
>> working on the patch set but for me all of these approaches look awfully
>> complicated.
>>
>> I'll throw my own suggestion and apologize if this has been already
>> suggested and discarded: return-to-AEP.
>>
>> My idea is to do just a small extension to SGX AEX handling. At the
>> moment hardware will RAX, RBX and RCX with ERESUME parameters. We can
>> fill extend this by filling other three spare registers with exception
>> information.
>>
>> AEP handler can then do whatever it wants to do with this information
>> or just do ERESUME.
> 
> A correction here. In practice this will add a requirement to have a bit
> more complicated AEP code (check the regs for exceptions) than before
> and not just bytes for ENCLU.
> 
> e.g. AEP handler should be along the lines
> 
> 1. #PF (or #UD or) happens. Kernel fills the registers when it cannot
>     handle the exception and returns back to user space i.e. to the
>     AEP handler.
> 2. Check the registers containing exception information. If they have
>     been filled, take whatever actions user space wants to take.
> 3. Otherwise, just ERESUME.
> 
>  From my point of view this is making the AEP parameter useful. Its
> standard use is just weird (always point to a place just containing
> ENCLU bytes, why the heck it even exists).

I like this solution. Keeps things simple. One question: when an 
exception occurs, how does the kernel know whether to set special 
registers or send a signal?

--
Jethro Beekman | Fortanix



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