linux-kernel - Re: [RFC PATCH v1 3/3] selftests/x86: Augment SGX selftest to test new __vdso_sgx_enter

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Message-ID: <CALCETrWcJw7+tSni3zp4kO=r6gVGkVDnc2477fPg0ErUzvJAKg@mail.gmail.com>
Date:   Tue, 23 Apr 2019 12:07:26 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Cedric Xing <cedric.xing@...el.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        linux-sgx@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave <dave.hansen@...el.com>, nhorman@...hat.com,
        npmccallum@...hat.com, Serge <serge.ayoun@...el.com>,
        Shay <shay.katz-zamir@...el.com>,
        Haitao <haitao.huang@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kai <kai.svahn@...el.com>, Borislav Petkov <bp@...en8.de>,
        Josh Triplett <josh@...htriplett.org>,
        Kai <kai.huang@...el.com>, David Rientjes <rientjes@...gle.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Subject: Re: [RFC PATCH v1 3/3] selftests/x86: Augment SGX selftest to test
 new __vdso_sgx_enter_enclave() and its callback interface

On Tue, Apr 23, 2019 at 11:59 AM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Mon, Apr 22, 2019 at 06:29:06PM -0700, Andy Lutomirski wrote:
> > What's not tested here is running this code with EFLAGS.TF set and
> > making sure that it unwinds correctly.  Also, Jarkko, unless I missed
> > something, the vDSO extable code likely has a bug.  If you run the
> > instruction right before ENCLU with EFLAGS.TF set, then do_debug()
> > will eat the SIGTRAP and skip to the exception handler.  Similarly, if
> > you put an instruction breakpoint on ENCLU, it'll get skipped.  Or is
> > the code actually correct and am I just remembering wrong?
>
> The code is indeed broken, and I don't see a sane way to make it not
> broken other than to never do vDSO fixup on #DB or #BP.  But that's
> probably the right thing to do anyways since an attached debugger is
> likely the intended recipient the 99.9999999% of the time.
>
> The crux of the matter is that it's impossible to identify whether or
> not a #DB/#BP originated from within an enclave, e.g. an INT3 in an
> enclave will look identical to an INT3 at the AEP.  Even if hardware
> provided a magic flag, #DB still has scenarios where the intended
> recipient is ambiguous, e.g. data breakpoint encountered in the enclave
> but on an address outside of the enclave, breakpoint encountered in the
> enclave and a code breakpoint on the AEP, etc...

Ugh.  It sounds like ignoring the fixup for #DB is the right call.
But what happens if the enclave contains an INT3 or ICEBP instruction?
 Are they magically promoted to #GP, perhaps?

As a maybe possible alternative, if we made it so that the AEX address
was not the same as the ENCLU, could we usefully distinguish these
exceptions based on RIP?  I suppose it's also worth considering
whether page faults from *inside* the enclave should result in SIGSEGV
or result in a fixup.  We certainly want page faults from the ENCLU
instruction itself to get fixed up, but maybe we want most exceptions
inside the enclave to work a bit differently.  Of course, if we do
this, we need to make sure that the semantics of returning from the
signal handler are reasonable.