Re: [RFC PATCH 1/2] kfence: Allow re-enabling KFENCE after system startup

[Tianchen Ding]

On 2022/3/5 02:13, Marco Elver wrote:
> On Thu, 3 Mar 2022 at 04:15, Tianchen Ding <dtcccc@xxxxxxxxxxxxxxxxx> wrote:
> > If once KFENCE is disabled by:
> > echo 0 > /sys/module/kfence/parameters/sample_interval
> > KFENCE could never be re-enabled until next rebooting.
> >
> > Allow re-enabling it by writing a positive num to sample_interval.
> >
> > Signed-off-by: Tianchen Ding <dtcccc@xxxxxxxxxxxxxxxxx>
>
> The only problem I see with this is if KFENCE was disabled because of
> a KFENCE_WARN_ON(). See below.
>
> > ---
> >   mm/kfence/core.c | 16 ++++++++++++++--
> >   1 file changed, 14 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> > index 13128fa13062..19eb123c0bba 100644
> > --- a/mm/kfence/core.c
> > +++ b/mm/kfence/core.c
> > @@ -55,6 +55,7 @@ EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */
> >   #endif
> >   #define MODULE_PARAM_PREFIX "kfence."
> >
> > +static int kfence_enable_late(void);
> >   static int param_set_sample_interval(const char *val, const struct kernel_param *kp)
> >   {
> >          unsigned long num;
> > @@ -65,10 +66,11 @@ static int param_set_sample_interval(const char *val, const struct kernel_param
> >
> >          if (!num) /* Using 0 to indicate KFENCE is disabled. */
> >                  WRITE_ONCE(kfence_enabled, false);
> > -       else if (!READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
> > -               return -EINVAL; /* Cannot (re-)enable KFENCE on-the-fly. */
> >
> >          *((unsigned long *)kp->arg) = num;
> > +
> > +       if (num && !READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
>
> Should probably have an 'old_sample_interval = *((unsigned long
> *)kp->arg)' somewhere before, and add a '&& !old_sample_interval',
> because if old_sample_interval!=0 then KFENCE was disabled due to a
> KFENCE_WARN_ON(). Also in this case, it should return -EINVAL. So you
> want a flow like this:
>
> old_sample_interval = ...;
> ...
> if (num && !READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
>    return old_sample_interval ? -EINVAL : kfence_enable_late();
> ...

Because sample_interval will used by delayed_work, we must put setting 
sample_interval before enabling KFENCE.
So the order would be:

old_sample_interval = sample_interval;
sample_interval = num;
if (...) kfence_enable_late();

This may be bypassed after KFENCE_WARN_ON() happens, if we first write 
0, and then write 100 to it.

How about this one:

	if (ret < 0)
		return ret;

+	/* Cannot set sample_interval after KFENCE_WARN_ON(). */
+	if (unlikely(*((unsigned long *)kp->arg) && !READ_ONCE(kfence_enabled)))
+		return -EINVAL;
+
	if (!num) /* Using 0 to indicate KFENCE is disabled. */
		WRITE_ONCE(kfence_enabled, false);

> Thanks,
> -- Marco