linux-kernel - Re: RFC: userspace exception fixups

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Date:   Tue, 6 Nov 2018 13:00:49 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rich Felker <dalias@...c.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jethro Beekman <jethro@...tanix.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Florian Weimer <fweimer@...hat.com>,
        Linux API <linux-api@...r.kernel.org>, X86 ML <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.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>,
        Carlos O'Donell <carlos@...hat.com>,
        adhemerval.zanella@...aro.org
Subject: Re: RFC: userspace exception fixups

On 11/6/18 12:12 PM, Andy Lutomirski wrote:
> True, but what if we have a nasty enclave that writes to memory just
> below SP *before* decrementing SP?

Yeah, that would be unfortunate.  If an enclave did this (roughly):

	1. EENTER
	2. Hardware sets eenter_hwframe->sp = %sp
	3. Enclave runs... wants to do out-call
	4. Enclave sets up parameters:
		memcpy(&eenter_hwframe->sp[-offset], arg1, size);
		...
	5. Enclave sets eenter_hwframe->sp -= offset

If we got a signal between 4 and 5, we'd clobber the copy of 'arg1' that
was on the stack.  The enclave could easily fix this by moving ->sp first.

But, this is one of those "fun" parts of the ABI that I think we need to
talk about.  If we do this, we also basically require that the code
which handles asynchronous exits must *not* write to the stack.  That's
not hard because it's typically just a single ERESUME instruction, but
it *is* a requirement.

It means fun stuff like that you absolutely can't just async-exit to C code.