Re: [PATCH v4.1 4/4] locking/lockdep: Add kasan_check_byte() check in lock_acquire()

[Andrey Konovalov]

On Fri, Feb 14, 2025 at 8:53 PM 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 print out a KASAN report before
> any lockdep splat, if any.
>
> Suggested-by: Marco Elver <elver@xxxxxxxxxx>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
>  kernel/locking/lockdep.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 8436f017c74d..b15757e63626 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 +5831,14 @@ 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 and other
> +        * memory errors.
> +        */
> +       kasan_check_byte(lock);
> +
>         if (unlikely(!lockdep_enabled())) {
>                 /* XXX allow trylock from NMI ?!? */
>                 if (lockdep_nmi() && !trylock) {
> --
> 2.48.1
>

I wonder if kasan_check_read/write() would be a better fit here. Those
are intended for the compiler-based modes and are no-op for HW_TAGS.
But I assume lockdep will access this lock variable anyway, so HW_TAGS
will detect memory errors.

On the other hand, detecting a bug earlier is better, so
kasan_check_byte() seems the better choice. And lockdep is not
intended to be fast / used on production anyway, so the extra
instructions added by kasan_check_byte() for HW_TAGS don't matter.

I guess we can change this later, if there's ever a reason to do so.

Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>