CVE-2026-98789 bpf: Fix objects stuck in free_by_rcu_ttrace
A flaw in the Linux kernel BPF subsystem allows unbounded memory retention, leading to resource exhaustion and denial of service.
- 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-2026-98789?
CVE-2026-98789 is a denial of service vulnerability in Linux, published on 18 Oct 2026. 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:
bpf: Fix objects stuck in free_by_rcu_ttrace
do_call_rcu_ttrace() returns early when call_rcu_ttrace_in_progress is set and leaves the objects in free_by_rcu_ttrace. __free_rcu() frees waiting_for_gp_ttrace only and clears the flag. Hence the objects that free_bulk() or __free_by_rcu() added while RCU tasks trace GP was in flight stay in free_by_rcu_ttrace until free_bulk() or alloc_bulk() is called for the same bpf_mem_cache again, which may never happen. The number of such objects is not bounded.
Turn call_rcu_ttrace_in_progress into three states: 0 - idle 1 - __free_rcu() is queued 2 - __free_rcu() is queued and free_by_rcu_ttrace got more objects since
do_call_rcu_ttrace() sets 2. __free_rcu() does cmpxchg(1 -> 0) and starts the next GP when it fails. It cannot clear the flag first and check free_by_rcu_ttrace later, since bpf_mem_alloc_destroy() frees bpf_mem_cache without waiting for RCU callbacks when the flag is zero.
Now __free_rcu() queues itself, so the one that didn't see 'draining' may do call_rcu_tasks_trace() after rcu_barrier_tasks_trace() in free_mem_alloc(). Queue it under rcu_read_lock() and do synchronize_rcu() before the barriers. Calling rcu_barrier_tasks_trace() twice works too, but creating and destroying hash maps in a loop on many cpus slows down to one free_mem_alloc() per GP and kworkers pile up.
Which versions are affected by CVE-2026-98789?
How do I fix CVE-2026-98789?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-98789 being exploited?
There is no known exploitation of CVE-2026-98789 in the wild and no public exploit code.
Sources
Also known as AZL-109236, BELL-CVE-2026-98789, DEBIAN-CVE-2026-98789, GHSA-49x9-888x-jjxr, UBUNTU-CVE-2026-98789.