Re: [PATCH mm 1/2] kasan: don't use read-only static keys

[Marco Elver <elver@google.com>]

On Wed, 9 Dec 2020 at 19:57, Kees Cook <keescook@chromium.org> wrote:
>
> On Wed, Dec 09, 2020 at 07:49:36PM +0100, Marco Elver wrote:
> > On Wed, 9 Dec 2020 at 19:24, Andrey Konovalov <andreyknvl@google.com> wrote:
> > > __ro_after_init static keys are incompatible with usage in loadable kernel
> > > modules and cause crashes. Don't use those, use normal static keys.
> > >
> > > Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> >
> > Reviewed-by: Marco Elver <elver@google.com>
> >
> > > ---
> > >
> > > This fix can be squashed into
> > > "kasan: add and integrate kasan boot parameters".
> > >
> > > ---
> > >  mm/kasan/hw_tags.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
> > > index c91f2c06ecb5..55bd6f09c70f 100644
> > > --- a/mm/kasan/hw_tags.c
> > > +++ b/mm/kasan/hw_tags.c
> > > @@ -43,11 +43,11 @@ static enum kasan_arg_stacktrace kasan_arg_stacktrace __ro_after_init;
> > >  static enum kasan_arg_fault kasan_arg_fault __ro_after_init;
> > >
> > >  /* Whether KASAN is enabled at all. */
> > > -DEFINE_STATIC_KEY_FALSE_RO(kasan_flag_enabled);
> > > +DEFINE_STATIC_KEY_FALSE(kasan_flag_enabled);
> >
> > Side-node: This appears to be just a bad interface; I think the macro
> > DEFINE_STATIC_KEY_FALSE_RO() is error-prone, if it can't be guaranteed
> > that this is always safe, since the presence of the macro encourages
> > its use and we'll inevitably run into this problem again.
> >
> > >  EXPORT_SYMBOL(kasan_flag_enabled);
> >
> > DEFINE_STATIC_KEY_FALSE_RO() + EXPORT_SYMBOL() is an immediate bug.
> > Given its use has not increased substantially since its introduction,
> > it may be safer to consider its removal.
>
> Right -- it seems the export is the problem, not the RO-ness. What is
> actually trying to change the flag after __init?

It seems to want to add it to a list on module loads:
https://lore.kernel.org/lkml/20201208125129.GY2414@hirez.programming.kicks-ass.net/

-- Marco