linux-kernel - Re: RFC: userspace exception fixups

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Date:   Tue, 20 Nov 2018 10:09:44 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jethro Beekman <jethro@...tanix.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,
        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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: RFC: userspace exception fixups

On Tue, Nov 20, 2018 at 12:11:33PM +0200, Jarkko Sakkinen wrote:
> On Mon, Nov 19, 2018 at 09:00:08AM -0800, Andy Lutomirski wrote:
> > On Mon, Nov 19, 2018 at 8:02 AM Jarkko Sakkinen
> > <jarkko.sakkinen@...ux.intel.com> wrote:
> > >
> > > On Mon, Nov 19, 2018 at 07:29:36AM -0800, Andy Lutomirski wrote:
> > > > 1. The kernel needs some way to know *when* to apply this fixup.
> > > > Decoding the instruction stream and doing it to all exceptions that
> > > > hit an ENCLU instruction seems like a poor design.
> > >
> > > I'm not sure why you would ever need to do any type of fixup as the idea
> > > is to just return to AEP i.e. from chosen exceptions (EPCM, #UD) the AEP
> > > would work the same way as for exceptions that the kernel can deal with
> > > except filling the exception information to registers.
> > 
> > Sure, but how does the kernel know when to do that and when to send a
> > signal?  I don't really like decoding the instruction stream to figure
> > it out.
> 
> Hmm... why you have to decode instruction stream to find that out? Would
> just depend on exception type (#GP with EPCM, #UD).

#PF w/ PFEC_SGX is the only exception that indicates a fault is related
to SGX.  Theoretically we could avoid decoding by using a magic value
for the AEP itself and doing even more magic fixup, but that wouldn't
help for faults that occur on EENTER, which can be generic #GPs due to
loss of EPC on SGX1 systems. 

> Or are you saying
> that kernel should need to SIGSEGV if there is in fact ENCLU so that
> there is no infinite trap loop? Sorry, I'm a bit lost here that where
> does this decoding requirement comes from in the first place. I
> understand how it is used in Sean's proposal...
> 
> Anyway, this option can be probably discarded without further
> consideration because apparently single stepping can cause #DB SS fault
> if AEP handler is anything else than a single instruction.

Not that it matters, but we could satisfy the "one instruction"
requirement if the fixup changed RIP to point at an ENCLU for #DBs.

> For me it seems that by ruling out options, vDSO option is what is
> left. I don't like it but at least it works...
> 
> /Jarkko