Re: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor

From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
To: Florian Weimer <fweimer@redhat.com>, Solar Designer <solar@openwall.com>
Cc: Pavel Machek <pavel@ucw.cz>,
	kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-fsdevel@vger.kernel.org, linux-integrity@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-security-module@vger.kernel.org, oleg@redhat.com,
	x86@kernel.org, luto@kernel.org, David.Laight@ACULAB.COM,
	mark.rutland@arm.com, mic@digikod.net,
	Rich Felker <dalias@libc.org>
Subject: Re: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor
Date: Wed, 23 Sep 2020 18:53:21 -0500	[thread overview]
Message-ID: <1a7c9989-fb98-20f7-c0d9-2261aa50d967@linux.microsoft.com> (raw)
In-Reply-To: <87wo0ko8v0.fsf@oldenburg2.str.redhat.com>

On 9/23/20 9:39 AM, Florian Weimer wrote:
> * Solar Designer:
> 
>> While I share my opinion here, I don't mean that to block Madhavan's
>> work.  I'd rather defer to people more knowledgeable in current userland
>> and ABI issues/limitations and plans on dealing with those, especially
>> to Florian Weimer.  I haven't seen Florian say anything specific for or
>> against Madhavan's proposal, and I'd like to.  (Have I missed that?)
> 
> There was a previous discussion, where I provided feedback (not much
> different from the feedback here, given that the mechanism is mostly the
> same).
> 
> I think it's unnecessary for the libffi use case.  Precompiled code can
> be loaded from disk because the libffi trampolines are so regular.  On
> most architectures, it's not even the code that's patched, but some of
> the data driving it, which happens to be located on the same page due to
> a libffi quirk.
> 
> The libffi use case is a bit strange anyway: its trampolines are
> type-generic, and the per-call adjustment is data-driven.  This means
> that once you have libffi in the process, you have a generic
> data-to-function-call mechanism available that can be abused (it's even
> fully CET compatible in recent versions).  And then you need to look at
> the processes that use libffi.  A lot of them contain bytecode
> interpreters, and those enable data-driven arbitrary code execution as
> well.  I know that there are efforts under way to harden Python, but
> it's going to be tough to get to the point where things are still
> difficult for an attacker once they have the ability to make mprotect
> calls.
> 
> It was pointed out to me that libffi is doing things wrong, and the
> trampolines should not be type-generic, but generated so that they match
> the function being called.  That is, the marshal/unmarshal code would be
> open-coded in the trampoline, rather than using some generic mechanism
> plus run-time dispatch on data tables describing the function type.
> That is a very different design (and typically used by compilers (JIT or
> not JIT) to implement native calls).  Mapping some code page with a
> repeating pattern would no longer work to defeat anti-JIT measures
> because it's closer to real JIT.  I don't know if kernel support could
> make sense in this context, but it would be a completely different
> patch.
> 
> Thanks,
> Florian
> 
Hi Florian,

I am making myself familiar with anti-JIT measures before I can respond
to this comment. Bear with me. I will also respond to the above
libffi comment.

Madhavan