linux-kernel - Re: [PATCH] mm: export copy_to_kernel

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Message-ID: <Zxiev9UaoUlI1xs9@infradead.org>
Date: Tue, 22 Oct 2024 23:59:11 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Marco Elver <elver@...gle.com>
Cc: Christoph Hellwig <hch@...radead.org>, Arnd Bergmann <arnd@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sabyrzhan Tasbolatov <snovitoll@...il.com>,
	Andrey Konovalov <andreyknvl@...il.com>,
	Arnd Bergmann <arnd@...db.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: export copy_to_kernel_nofault

On Wed, Oct 23, 2024 at 08:57:02AM +0200, Marco Elver wrote:
> On Wed, 23 Oct 2024 at 08:53, Christoph Hellwig <hch@...radead.org> wrote:
> >
> > On Fri, Oct 18, 2024 at 03:11:09PM +0000, Arnd Bergmann wrote:
> > > From: Arnd Bergmann <arnd@...db.de>
> > >
> > > This symbol is now used on the kasan test module, so it needs to be
> > > exported.
> > >
> > > ERROR: modpost: "copy_to_kernel_nofault" [mm/kasan/kasan_test.ko] undefined!
> >
> > Meh, it would be great not to export internal helpers just because
> > someone wants to test them.  Please just mark that test built-in only
> > instead.
> 
> We have EXPORT_SYMBOL_IF_KUNIT. See include/kunit/visibility.h -
> that's more appropriate, and also adjust kasan_test.c to do
> MODULE_IMPORT_NS(EXPORTED_FOR_KUNIT_TESTING).

Thats a little better, but at least in this case I still think it is
a very bad idea.  copy_to_kernel_nofault is a perfect vector for
exploit code to probe writing to kernel address without causing faults
so it really should never ever be exported.