# CVE-2026-98642: bpf: Fix UAF due to concurrent consumption of ttrace lists in alloc_bulk

> A race condition in the Linux kernel BPF memory allocator allows local attackers to trigger a use-after-free, potentially leading to code execution or privilege escalation.

- **Severity:** Not scored
- **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:** Yes
- **Published:** 18 Oct 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-109311, BELL-CVE-2026-98642, DEBIAN-CVE-2026-98642, GHSA-9437-xj9f-2c6g, UBUNTU-CVE-2026-98642

## What is CVE-2026-98642?

CVE-2026-98642 is an use after free 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 UAF due to concurrent consumption of ttrace lists in alloc_bulk

Syzkaller repeatedly triggered UAF splats related to nodes in
waiting_for_gp_ttrace within the bpf memalloc:

BUG: KASAN: slab-use-after-free in llist_del_first+0x85/0x110 lib/llist.c:61
Read of size 8 at addr ffff8881572cd080 by task syz.4.470/5112
 ...
 llist_del_first+0x85/0x110 lib/llist.c:61
 alloc_bulk+0x193/0x460 kernel/bpf/memalloc.c:229
 bpf_mem_refill+0x386/0x560 kernel/bpf/memalloc.c:436

Freed by task 14:
 ...
 __free_rcu kernel/bpf/memalloc.c:281 [inline]
 __free_rcu_tasks_trace+0x48/0xd0 kernel/bpf/memalloc.c:291
 rcu_tasks_invoke_cbs+0x1ec/0x3e0 kernel/rcu/tasks.h:571
 rcu_tasks_one_gp+0x13d/0x220 kernel/rcu/tasks.h:621
 rcu_tasks_kthread+0xf3/0x120 kernel/rcu/tasks.h:651

The reason is that the UAF occurs after the RCU Tasks Trace GP expires:
when the __free_rcu() callback runs, there is no synchronization
protecting llist_del_all() against concurrent alloc_bulk() operating on
waiting_for_gp_ttrace, leading to the race condition below:

CPU0                                           CPU1
                                               __free_rcu (RCU Tasks Trace callback)
alloc_bulk
  llist_del_first(&c->waiting_for_gp_ttrace)
    entry = smp_load_acquire(&head->first);
    do {
      if (entry == NULL)
        return NULL;
                                               free_all(llist_del_all(&c->waiting_for_gp_ttrace))
                                                 llist_for_each_safe(pos, t, llnode)
                                                   free_one(pos);
      next = READ_ONCE(entry->next); <-- trigger UAF
    } while (!try_cmpxchg(&head->first, &entry, next));

In addition, there is also a theoretical race condition on the
free_by_rcu_ttrace list. This race requires two preconditions: an
in-flight Tasks Trace GP keeping c->call_rcu_ttrace_in_progress == 1,
and concurrent cross-CPU frees repopulating c->free_by_rcu_ttrace with
new nodes. Under these conditions, the following scenario triggers UAF:

// CPU0
// irq work is still busy (on PREEMPT_RT)
alloc_bulk()
  llist_del_first(&c->free_by_rcu_ttrace)
    entry = smp_load_acquire(&head->first);
    do {
      if (entry == NULL)
        return NULL;

        // CPU1
        bpf_mem_alloc_destroy()
          WRITE_ONCE(c->draining, true)
          // wait for CPU0
          irq_work_sync()

                // CPU2
                do_call_rcu_ttrace(tgt(CPU0))
                  if (c->draining) {
                    llist_del_all(&c->free_by_rcu_ttrace)
                    free_all()
                  }

// CPU0 continue
      next = READ_ONCE(entry->next); <-- trigger UAF
    while (!try_cmpxchg(&head->first, &entry, next));

Fix this by introducing a raw spinlock to synchronize the concurrent
consumption on waiting_for_gp_ttrace and free_by_rcu_ttrace.

## Which versions are affected by CVE-2026-98642?


## How do I fix CVE-2026-98642?

Apply the vendor's patch.

## Is CVE-2026-98642 being exploited?

There is no known exploitation of CVE-2026-98642 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98642 and fix it if it is affected.

What it is: CVE-2026-98642 is an use after free 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.
Exploitation: There is no known exploitation of CVE-2026-98642 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-98642/ (Markdown: https://insights.sbomify.com/CVE-2026-98642/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98642
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98642
- OSV: https://osv.dev/vulnerability/CVE-2026-98642

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98642/
Summaries are generated and may contain mistakes; the official description above is the record.