linux-kernel - Re: [PATCH v2] mm: unexport globally copy_to_kernel

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Message-Id: <20250622112014.76bdd8929ecdb1c1fb3015b5@linux-foundation.org>
Date: Sun, 22 Jun 2025 11:20:14 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Sabyrzhan Tasbolatov <snovitoll@...il.com>
Cc: andreyknvl@...il.com, arnd@...db.de, david@...hat.com,
 dvyukov@...gle.com, elver@...gle.com, glider@...gle.com, hch@...radead.org,
 kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, ryabinin.a.a@...il.com, vincenzo.frascino@....com
Subject: Re: [PATCH v2] mm: unexport globally copy_to_kernel_nofault

On Sun, 22 Jun 2025 19:11:42 +0500 Sabyrzhan Tasbolatov <snovitoll@...il.com> wrote:

> `copy_to_kernel_nofault()` is an internal helper which should not be
> visible to loadable modules – exporting it would give exploit code a
> cheap oracle to probe kernel addresses.  Instead, keep the helper
> un-exported and compile the kunit case that exercises it only when
> `mm/kasan/kasan_test.o` is linked into vmlinux.

The recent 707f853d7fa3 ("module: Provide
EXPORT_SYMBOL_GPL_FOR_MODULES() helper") quietly added a thing which
might be useful here.  As far as I understand it, this will permit us
to export copy_to_kernel_nofault to kasan_test_c.o and to nothing else.

"might".  It depends on how "exploit code" might get hold of the
symbol.  Perhaps you/we can discuss this further.  Is the problem that
copy_to_kernel_nofault() is non-static?  Or it the problem that
"exploit code" is itself a kernel module?

In other words, a fuller investigation of how this export presently benefits
exploiters would help us understand how much
EXPORT_SYMBOL_GPL_FOR_MODULES() will improve the situation.