CVE-2026-98084 bpf: backtracking shouldn't clear outer frame R1-R5 for callbacks
A flaw in the Linux kernel BPF verifier allows loading unsafe programs, potentially leading to privilege escalation or code execution.
- 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-2026-98084?
CVE-2026-98084 is a vulnerability in Linux, published on 25 Sep 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: backtracking shouldn't clear outer frame R1-R5 for callbacks
When processing calls to bpf_loop() verifier marks R1 (and R4) as precise. R1 tracks loop iterations number and because of the 'callback_depth < R1' mechanics in check_helper_call() must be marked precise. However, precision propagation for R1 was broken, when bpf_loop() call was verified on a second iteration.
Consider the following verification trace: - main: bpf_loop(nr_loops, callback ...) - callback: BPF_EXIT - main: bpf_loop(nr_loops, callback ...) - ...
While the first visit of the call to bpf_loop() propagated R1 precision as expected, the second call to mark_chain_precision() in the check_helper_call() set R1, but it was immediately reset when backtrack_insn() processed preceding BPF_EXIT in the loop deleted in this patch.
Because of that, the second visit of the call to bpf_loop() injected checkpoint with R1 not marked as precise. Which could trick the verifier into accepting unsafe programs. See the next patch for an example of such program.
Commit is structured in a way to minimize conflicts when 'bpf' would be eventually merged with 'bpf-next'.
Which versions are affected by CVE-2026-98084?
How do I fix CVE-2026-98084?
Apply the vendor's patch.
Is CVE-2026-98084 being exploited?
There is no known exploitation of CVE-2026-98084 in the wild and no public exploit code.
Sources
Also known as AZL-104294, BELL-CVE-2026-98084, DEBIAN-CVE-2026-98084, GHSA-hcwg-2fch-4v4c, UBUNTU-CVE-2026-98084.