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

[Tianchen Ding]

On 2022/3/5 13:26, Tianchen Ding wrote:
> 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);

Hmm...
I found KFENCE_WARN_ON() may be called when sample_interval==0. (e.g., 
kfence_guarded_free())
So it's better to add a bool.

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index ae69b2a113a4..c729be0207e8 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -38,14 +38,17 @@
 #define KFENCE_WARN_ON(cond) 
         \
 	({                                                                     \
 		const bool __cond = WARN_ON(cond);                             \
-		if (unlikely(__cond))                                          \
+		if (unlikely(__cond)) {                                        \
 			WRITE_ONCE(kfence_enabled, false);                     \
+			disabled_by_warn = true;                               \
+		}                                                              \
 		__cond;                                                        \
 	})

 /* === Data 
================================================================= */

 static bool kfence_enabled __read_mostly;
+static bool disabled_by_warn __read_mostly;

 unsigned long kfence_sample_interval __read_mostly = 
CONFIG_KFENCE_SAMPLE_INTERVAL;
 EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */
@@ -70,7 +73,7 @@ static int param_set_sample_interval(const char *val, 
const struct kernel_param
 	*((unsigned long *)kp->arg) = num;

 	if (num && !READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
-		return kfence_enable_late();
+		return disabled_by_warn ? -EINVAL : kfence_enable_late();
 	return 0;
 }