Re: [PATCH v4 4/4] locking/lockdep: Add kasan_check_byte() check in lock_acquire()

[Waiman Long]

On 2/14/25 9:44 AM, Marco Elver wrote:
> On Fri, 14 Feb 2025 at 15:09, Waiman Long <llong@xxxxxxxxxx> wrote:
> > On 2/14/25 5:44 AM, Marco Elver wrote:
> > > On Thu, 13 Feb 2025 at 21:02, Waiman Long <longman@xxxxxxxxxx> wrote:
> > > > KASAN instrumentation of lockdep has been disabled as we don't need
> > > > KASAN to check the validity of lockdep internal data structures and
> > > > incur unnecessary performance overhead. However, the lockdep_map pointer
> > > > passed in externally may not be valid (e.g. use-after-free) and we run
> > > > the risk of using garbage data resulting in false lockdep reports. Add
> > > > kasan_check_byte() call in lock_acquire() for non kernel core data
> > > > object to catch invalid lockdep_map and abort lockdep processing if
> > > > input data isn't valid.
> > > >
> > > > Suggested-by: Marco Elver <elver@xxxxxxxxxx>
> > > > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > > Reviewed-by: Marco Elver <elver@xxxxxxxxxx>
> > >
> > > but double-check if the below can be simplified.
> > >
> > > > ---
> > > >    kernel/locking/lock_events_list.h |  1 +
> > > >    kernel/locking/lockdep.c          | 14 ++++++++++++++
> > > >    2 files changed, 15 insertions(+)
> > > >
> > > > diff --git a/kernel/locking/lock_events_list.h b/kernel/locking/lock_events_list.h
> > > > index 9ef9850aeebe..bed59b2195c7 100644
> > > > --- a/kernel/locking/lock_events_list.h
> > > > +++ b/kernel/locking/lock_events_list.h
> > > > @@ -95,3 +95,4 @@ LOCK_EVENT(rtmutex_deadlock)  /* # of rt_mutex_handle_deadlock()'s    */
> > > >    LOCK_EVENT(lockdep_acquire)
> > > >    LOCK_EVENT(lockdep_lock)
> > > >    LOCK_EVENT(lockdep_nocheck)
> > > > +LOCK_EVENT(lockdep_kasan_fail)
> > > > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > > > index 8436f017c74d..98dd0455d4be 100644
> > > > --- a/kernel/locking/lockdep.c
> > > > +++ b/kernel/locking/lockdep.c
> > > > @@ -57,6 +57,7 @@
> > > >    #include <linux/lockdep.h>
> > > >    #include <linux/context_tracking.h>
> > > >    #include <linux/console.h>
> > > > +#include <linux/kasan.h>
> > > >
> > > >    #include <asm/sections.h>
> > > >
> > > > @@ -5830,6 +5831,19 @@ void lock_acquire(struct lockdep_map *lock, unsigned int subclass,
> > > >           if (!debug_locks)
> > > >                   return;
> > > >
> > > > +       /*
> > > > +        * As KASAN instrumentation is disabled and lock_acquire() is usually
> > > > +        * the first lockdep call when a task tries to acquire a lock, add
> > > > +        * kasan_check_byte() here to check for use-after-free of non kernel
> > > > +        * core lockdep_map data to avoid referencing garbage data.
> > > > +        */
> > > > +       if (unlikely(IS_ENABLED(CONFIG_KASAN) &&
> > > This is not needed - kasan_check_byte() will always return true if
> > > KASAN is disabled or not compiled in.
> > I added this check because of the is_kernel_core_data() call.
> > > > +                    !is_kernel_core_data((unsigned long)lock) &&
> > > Why use !is_kernel_core_data()? Is it to improve performance?
> > Not exactly. In my testing, just using kasan_check_byte() doesn't quite
> > work out. It seems to return false positive in some cases causing
> > lockdep splat. I didn't look into exactly why this happens and I added
> > the is_kernel_core_data() call to work around that.
> Globals should have their shadow memory unpoisoned by default, so
> that's definitely odd.
>
> Out of curiosity, do you have such a false positive splat? Wondering
> which data it's accessing. Maybe that'll tell us more about what's
> wrong.

Will do more investigation about this and let you know the result.

Cheers,
Longman

>