CVE-2025-21899

In the Linux kernel, the following vulnerability has been resolved:

tracing: Fix bad hist from corrupting named_triggers list

The following commands causes a crash:

~# cd /sys/kernel/tracing/events/rcu/rcu_callback ~# echo hist:name=bad:keys=common_pid:onmax(bogus).save(common_pid) > trigger bash: echo: write error: Invalid argument ~# echo hist:name=bad:keys=common_pid > trigger

Because the following occurs:

event_trigger_write() { trigger_process_regex() { event_hist_trigger_parse() {

  data = event_trigger_alloc(..);

  event_trigger_register(.., data) {
    cmd_ops->reg(.., data, ..) [hist_register_trigger()] {
      data->ops->init() [event_hist_trigger_init()] {
        save_named_trigger(name, data) {
          list_add(&data->named_list, &named_triggers);
        }
      }
    }
  }

  ret = create_actions(); (return -EINVAL)
  if (ret)
    goto out_unreg;

[..] ret = hist_trigger_enable(data, …) { list_add_tail_rcu(&data->list, &file->triggers); «<—- SKIPPED!!! (this is important!) [..] out_unreg: event_hist_unregister(.., data) { cmd_ops->unreg(.., data, ..) [hist_unregister_trigger()] { list_for_each_entry(iter, &file->triggers, list) { if (!hist_trigger_match(data, iter, named_data, false)) <- never matches continue; [..] test = iter; } if (test && test->ops->free) «<– test is NULL

        test->ops->free(test) [event_hist_trigger_free()] {
          [..]
          if (data->name)
            del_named_trigger(data) {
              list_del(&data->named_list);  <<<<-- NEVER gets removed!
            }
          }
       }
     }

     [..]
     kfree(data); <<<-- frees item but it is still on list

The next time a hist with name is registered, it causes an u-a-f bug and the kernel can crash.

Move the code around such that if event_trigger_register() succeeds, the next thing called is hist_trigger_enable() which adds it to the list.

A bunch of actions is called if get_named_trigger_data() returns false. But that doesnt need to be called after event_trigger_register(), so it can be moved up, allowing event_trigger_register() to be called just before hist_trigger_enable() keeping them together and allowing the file->triggers to be properly populated.

Weakness

The product dereferences a pointer that it expects to be valid but is NULL.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/435d2964af815aae456db554c62963b4515f19d0
• https://git.kernel.org/stable/c/43b254d46c740bf9dbe65709afa021dd726dfa99
• https://git.kernel.org/stable/c/5ae1b18f05ee2b849dc03b6c15d7da0c1c6efa77
• https://git.kernel.org/stable/c/6f86bdeab633a56d5c6dccf1a2c5989b6a5e323e
• https://git.kernel.org/stable/c/f1ae50cfb818ce1ac7a674406dfadb7653e2552d
• https://lists.debian.org/debian-lts-announce/2025/05/msg00045.html