Re: [PATCH] x86/uaccess: instrument copy_from_user_nmi()

[Kees Cook <keescook@chromium.org>]

On Thu, Oct 27, 2022 at 11:26:50AM -0700, Alexander Potapenko wrote:
> On Thu, Oct 27, 2022 at 1:05 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Wed, Oct 26, 2022 at 11:38:53AM -0700, Alexander Potapenko wrote:
> > > A bigger issue from the NMI perspective is probably
> > > having __msan_poison_alloca() inserted in every non-noinstr kernel
> > > function, because that hook may acquire the stackdepot lock.
> >
> > *urgghhh* that's broken, that must not be. There is a *TON* of NMI
> > functions that are non-noinstr.
> 
> __msan_poison_alloca() is guarded by kmsan_in_runtime(), which is
> currently implemented as:
> 
>   static __always_inline bool kmsan_in_runtime(void)
>   {
>           if ((hardirq_count() >> HARDIRQ_SHIFT) > 1)
>                   return true;
>           return kmsan_get_context()->kmsan_in_runtime;
>   }
> 
> I think the easiest way to fix the NMI situation would be adding "if
> in_nmi() return true"?

It might help to look through these threads:

https://lore.kernel.org/lkml/20220916135953.1320601-1-keescook@chromium.org/
https://lore.kernel.org/all/20220919201648.2250764-1-keescook@chromium.org/

I wandered around attempting to deal with in_nmi(), etc. And in
the end just drop the attempt to cover it. It's worth noting that
copy_from_user_nmi() exists on 1 architecture and has exactly 1
call-site...

-- 
Kees Cook