Re: [PATCH] tracing/user_events: Use kfree_rcu for enabler cleanup

[Beau Belgrave]

On Thu, Jun 25, 2026 at 06:02:03PM +0000, Tristan Madani wrote:
> From: Tristan Madani <tristan@xxxxxxxxxxxxxxxxxxx>
> 
> user_event_enabler_destroy() removes the enabler from an RCU-protected
> list via list_del_rcu() and then immediately frees it with kfree(). This
> can result in a concurrent reader in user_event_enabler_dup() accessing
> stale memory during fork, since the enabler list is traversed under
> rcu_read_lock().
> 
> The ENABLE_VAL_FREEING_BIT check in user_event_enabler_dup() is not
> sufficient to prevent this, as the enabler can be freed between the bit
> test and the subsequent pointer dereference.
> 
> Use kfree_rcu() to defer the free until after all RCU read-side critical
> sections complete.
> 
> Fixes: 7235759084a4 ("tracing/user_events: Use remote writes for event enablement")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Tristan Madani <tristan@xxxxxxxxxxxxxxxxxxx>
> ---
>  kernel/trace/trace_events_user.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
> index c4ba484f7b38b..72bcb429eb4f3 100644
> --- a/kernel/trace/trace_events_user.c
> +++ b/kernel/trace/trace_events_user.c
> @@ -109,6 +109,7 @@ struct user_event_enabler {
>  
>  	/* Track enable bit, flags, etc. Aligned for bitops. */
>  	unsigned long		values;
> +	struct rcu_head		rcu;
>  };
>  
>  /* Bits 0-5 are for the bit to update upon enable/disable (0-63 allowed) */
> @@ -404,7 +405,7 @@ static void user_event_enabler_destroy(struct user_event_enabler *enabler,
>  	/* No longer tracking the event via the enabler */
>  	user_event_put(enabler->event, locked);
>  
> -	kfree(enabler);
> +	kfree_rcu(enabler, rcu);
>  }
>  
>  static int user_event_mm_fault_in(struct user_event_mm *mm, unsigned long uaddr,
> -- 
> 2.47.3

See [1] as there are more issues than simply the enabler being freed via
RCU, there are lifetime aspects of the underlying user_event.

Thanks,
-Beau

1. https://lore.kernel.org/linux-trace-kernel/20260618222743.538915-1-michael.bommarito@xxxxxxxxx/