Re: [PATCH 5/5] modules: only allow symbol_get of EXPORT_SYMBOL_GPL modules

From: David Woodhouse <dwmw2@infradead.org>
To: Christoph Hellwig <hch@lst.de>,
	Luis Chamberlain <mcgrof@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Daniel Mack <daniel@zonque.org>,
	Haojian Zhuang <haojian.zhuang@gmail.com>,
	Robert Jarzmik <robert.jarzmik@free.fr>,
	Ulf Hansson <ulf.hansson@linaro.org>,
	Manuel Lauss <manuel.lauss@gmail.com>,
	Yangbo Lu <yangbo.lu@nxp.com>, Joshua Kinard <kumba@gentoo.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-arm-kernel@lists.infradead.org,
	open list <linux-kernel@vger.kernel.org>,
	linux-mmc@vger.kernel.org, netdev@vger.kernel.org,
	linux-rtc@vger.kernel.org, linux-modules@vger.kernel.org
Subject: Re: [PATCH 5/5] modules: only allow symbol_get of EXPORT_SYMBOL_GPL modules
Date: Wed, 18 Oct 2023 01:30:18 +0100	[thread overview]
Message-ID: <bf555c2a4df5196533b6e614cc57638004dfb426.camel@infradead.org> (raw)
In-Reply-To: <20230801173544.1929519-6-hch@lst.de>

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On Tue, 2023-08-01 at 19:35 +0200, Christoph Hellwig wrote:
> It has recently come to my attention that nvidia is circumventing the
> protection added in 262e6ae7081d ("modules: inherit
> TAINT_PROPRIETARY_MODULE") by importing exports from their proprietary
> modules into an allegedly GPL licensed module and then rexporting them.
> 
> Given that symbol_get was only ever intended for tightly cooperating
> modules using very internal symbols it is logical to restrict it to
> being used on EXPORT_SYMBOL_GPL and prevent nvidia from costly DMCA
> Circumvention of Access Controls law suites.

I'm all for insisting that everything be exported with
EXPORT_SYMBOL_GPL and nothing at all ever be exported with just
EXPORT_SYMBOL.

But if we're going to tolerate the core kernel still exporting some
stuff with EXPORT_SYMBOL, why isn't OK for a GPL-licensed module do to
the same? Even an *in-tree* GPL-licensed module now can't export
functionality with EXPORT_SYMBOL and have it used with symbol_get().

We're forced to *either* allow direct linking by non-GPL modules, or
allow symbol_get(), but not both?

> Fixes: 262e6ae7081d ("modules: inherit TAINT_PROPRIETARY_MODULE")

Hm, the condition we really need to fix *that* is "symbol_get() will
only import symbols from GPL-licensed modules", isn't it?

As long as that property is correctly transitive, why does the symbol
itself have to be EXPORT_SYMBOL_GPL instead of EXPORT_SYMBOL? Am I
missing another potential loophole?

I suppose there's now scope for a different type of shim which
*directly* imports an EXPORT_SYMBOL function in order to export it
again as EXPORT_SYMBOL_GPL and thus allow the GPL export to be found
with symbol_get()?

That's the *converse* of the problematic shim that was being used
before, and from a licensing point of view it seems fine... it's just
working around the unintended side-effects of this patch?

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