Re: [PATCH v2] mm: unexport globally copy_to_kernel_nofault

[Sabyrzhan Tasbolatov]

On Sun, Jun 22, 2025 at 11:20 PM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Sun, 22 Jun 2025 19:11:42 +0500 Sabyrzhan Tasbolatov <snovitoll@xxxxxxxxx> 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.

Thanks for letting me know about this new method.
I believe, the usage for our case is:
EXPORT_SYMBOL_GPL_FOR_MODULES(copy_to_kernel_nofault, "kasan_test");

>
> "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?

I haven't verified this, but theoretically, it's a handy
“write-anywhere-safely” ROP gadget.
Assume the attacker has already gained an arbitrary RW primitive
via a UAF/OOB bug. Instead of stitching together
prepare_kernel_cred() + commit_creds(), which is a common path
of using exported symbols to achieve privilege escalation.
This path needs two symbols and register juggling.
With exported copy_to_kernel_nofault() they can do this:

/* Pseudocode of exploit for a ROP stage running in kernel context */
        struct cred *cred = leaked_pointer;
        rop_call(copy_to_kernel_nofault, &cred->uid, &zero, 4)

copy_to_kernel_nofault() disables page-faults around the write,
so even if cred corupts a guard-page, the write will not crash.

>
> 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.
>

Please let me know if I should send v3 with using
EXPORT_SYMBOL_GPL_FOR_MODULES(copy_to_kernel_nofault, "kasan_test");