Re: [RFC PATCH v3 0/4] x86: Add exception fixup for SGX ENCLU

From: Andy Lutomirski <luto@kernel.org>
To: "Christopherson, Sean J" <sean.j.christopherson@intel.com>
Cc: Josh Triplett <josh@joshtriplett.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	X86 ML <x86@kernel.org>,
	Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Andrew Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-sgx@vger.kernel.org,
	Haitao Huang <haitao.huang@linux.intel.com>,
	Jethro Beekman <jethro@fortanix.com>,
	"Dr. Greg Wettstein" <greg@enjellic.com>
Subject: Re: [RFC PATCH v3 0/4] x86: Add exception fixup for SGX ENCLU
Date: Tue, 11 Dec 2018 09:58:19 -0800	[thread overview]
Message-ID: <CALCETrXOXh8Gb_d7NYLakx4w+JVEe79t11wFPLpy+5MEq9kq=A@mail.gmail.com> (raw)
In-Reply-To: <20181211165253.GB14731@linux.intel.com>

> On Dec 11, 2018, at 8:52 AM, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
>
>> On Tue, Dec 11, 2018 at 07:41:27AM -0800, Andy Lutomirski wrote:
>>
>>
>>>> On Dec 10, 2018, at 3:24 PM, Josh Triplett <josh@joshtriplett.org> wrote:
>>>>
>>>> On Mon, Dec 10, 2018 at 03:21:37PM -0800, Sean Christopherson wrote:
>>>> At that point I realized it's a hell of a lot easier to simply provide
>>>> an IOCTL via /dev/sgx that allows userspace to register a per-process
>>>> ENCLU exception handler.  At a high level, the basic idea is the same
>>>> as the vDSO approach: provide a hardcoded fixup handler for ENCLU and
>>>> attempt to fixup select unhandled exceptions that occurred in user code.
>>>
>>> So, on the one hand, this is *absolutely* much cleaner than the VDSO
>>> approach. On the other hand, this is global process state and has some
>>> of the same problems as a signal handler as a result.
>>
>> I liked the old version better for this reason
>
> This isn't fundamentally different than forcing all EENTER calls through
> the vDSO, which is also per-process.  Technically this is more flexible
> in that regard since userspace gets to choose where their one ENCLU gets
> to reside.  Userspace can have per-enclave entry flows so long as the
> actual ENLU[EENTER] is common, same as vDSO.

Right. The problem is that user libraries have a remarkably hard time
agreeing on where their one copy of anything lives.

>
>> and for another reason:
>> while this new one looks very very simple, it still has the hidden
>> complexity that the magic values written to registers in the event of an
>> exception are very much Linux specific.
>
> Definitely more magical, but not necessarily more difficult to document.
> It'd essentially be an extension of hardware's AEE/AEP behavior.
>
>> OTOH, the old approach clobbered more regs than needed, but that’s a easy fix.