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

[Waiman Long <llong@redhat.com>]

On 2/14/25 9:44 AM, Marco Elver wrote:
> On Fri, 14 Feb 2025 at 15:09, Waiman Long <llong@redhat.com> wrote:
>> On 2/14/25 5:44 AM, Marco Elver wrote:
>>> On Thu, 13 Feb 2025 at 21:02, Waiman Long <longman@redhat.com> 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@google.com>
>>>> Signed-off-by: Waiman Long <longman@redhat.com>
>>> Reviewed-by: Marco Elver <elver@google.com>
>>>
>>> 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

>