CVE-2026-90211 bpf, s390: Clear fetch destination on faulting arena atomic
A missing register clear in the Linux kernel s390x BPF JIT allows local attackers to read uninitialized memory during faulting atomic operations.
- 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-2026-90211?
CVE-2026-90211 is a vulnerability in Linux, published on 17 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, s390: Clear fetch destination on faulting arena atomic
Same missing register clear as on riscv64. A RMW atomic on an arena pointer is converted to BPF_PROBE_ATOMIC and gets an exception table entry, but bpf_jit_probe_atomic_pre() only fills in the arena base and the probe offset, leaving probe->reg at the -1 that bpf_jit_probe_init() set, which bpf_jit_probe_post() writes into the entry and ex_handler_bpf() then reads back as "there is nothing to clear".
That is right for a plain BPF_{ADD,AND,OR,XOR}, which only writes memory, but an RMW carrying BPF_FETCH also reads the old value into a register: src_reg for BPF_{ADD,AND,OR,XOR} | BPF_FETCH and BPF_XCHG, and r0 for BPF_CMPXCHG. So on a fault over an unmapped arena page the program resumes at the landing pad with whatever that register held before the atomic instead of the 0 that every other BPF_PROBE_* access delivers.
Fill probe->reg in from bpf_atomic_load_reg(). Unlike x86-64 and arm64, s390x does not report arena violations from its exception handler, so there is no access direction to correct here, only the missing register clear.
Which versions are affected by CVE-2026-90211?
How do I fix CVE-2026-90211?
Apply the vendor's patch.
Is CVE-2026-90211 being exploited?
There is no known exploitation of CVE-2026-90211 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-90211, DEBIAN-CVE-2026-90211, GHSA-xjxr-6fhh-44vq, UBUNTU-CVE-2026-90211.