CVE-2025-21899 tracing: Fix bad hist from corrupting named_triggers list
A use-after-free bug in the Linux kernel tracing subsystem allows local attackers to cause a system crash by corrupting the named triggers list.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-21899?
CVE-2025-21899 is a high-severity use after free vulnerability in Linux kernel, published on 1 Apr 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
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 doesn't 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: CWE-476
Which versions are affected by CVE-2025-21899?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-21899?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2025-21899 being exploited?
There is no known exploitation of CVE-2025-21899 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-69527, BELL-CVE-2025-21899, DEBIAN-CVE-2025-21899, GHSA-vp9f-g82m-h958, ROOT-OS-DEBIAN-11-CVE-2025-21899, ROOT-OS-UBUNTU-2204-CVE-2025-21899 and 2 more.