Subject: Re: [PATCH v5 2/8] lkdtm/powerpc: Add test to hijack a patch mapping

From: Kees Cook <[email-masked]>
Date: Wed, 11 Aug 2021 11:07:34 -0700

On Wed, Aug 11, 2021 at 12:57:00PM -0500, Christopher M. Riedl wrote:
> On Thu Aug 5, 2021 at 4:13 AM CDT, Christophe Leroy wrote:
> >
> >
> > Le 13/07/2021 à 07:31, Christopher M. Riedl a écrit :
> > > When live patching with STRICT_KERNEL_RWX the CPU doing the patching
> > > must temporarily remap the page(s) containing the patch site with +W
> > > permissions. While this temporary mapping is in use, another CPU could
> > > write to the same mapping and maliciously alter kernel text. Implement a
> > > LKDTM test to attempt to exploit such an opening during code patching.
> > > The test is implemented on powerpc and requires LKDTM built into the
> > > kernel (building LKDTM as a module is insufficient).
> > > 
> > > The LKDTM "hijack" test works as follows:
> > > 
> > >    1. A CPU executes an infinite loop to patch an instruction. This is
> > >       the "patching" CPU.
> > >    2. Another CPU attempts to write to the address of the temporary
> > >       mapping used by the "patching" CPU. This other CPU is the
> > >       "hijacker" CPU. The hijack either fails with a fault/error or
> > >       succeeds, in which case some kernel text is now overwritten.
> > > [...]
> > > +#if (IS_BUILTIN(CONFIG_LKDTM) && defined(CONFIG_STRICT_KERNEL_RWX) && \
> > > +	defined(CONFIG_PPC))
> >
> > I think this test shouldn't be limited to CONFIG_PPC and shouldn't be
> > limited to CONFIG_STRICT_KERNEL_RWX. It should be there all the time.

Agreed: if the machinery exists to provide this defense on even one
arch/config/whatever combo, I'd like LKDTM to test for it. This lets use
compare defenses across different combinations more easily, and means
folks must answer questions like "why doesn't $combination provide
$defense?"

> > Also why limiting it to IS_BUILTIN(CONFIG_LKDTM) ?
> 
> The test needs read_cpu_patching_addr() which definitely cannot be
> exposed outside of the kernel (ie. builtin).

FWIW, I'm okay with this. There isn't a solution that feels entirely
"right", so either a build-time requirement like this, or using an
exception for modules like this:

arch/x86/kernel/cpu/common.c:#if IS_MODULE(CONFIG_LKDTM)
arch/x86/kernel/cpu/common.c-EXPORT_SYMBOL_GPL(native_write_cr4);
arch/x86/kernel/cpu/common.c-#endif

I think neither is great. Another idea is maybe using a name-spaced
export, like:

EXPORT_SYMBOL_NS_GPL(native_write_cr4, LKDTM);

But that still means it gets exposed to malicious discovery, so probably
not.

I suspect the best is to just do the BUILTIN check, since building LKDTM
as a module on a _production_ kernel is rare if it exists at all. The
only downside is needing to completely reboot to perform updated tests,
but then, I frequently find myself breaking the kernel badly on bad
tests, so I have to reboot anyway. ;)

-Kees

-- 
Kees Cook