CVE-2026-98049bpf: zero extend the result of an arena 32-bit cmpxchg

Official description

In the Linux kernel, the following vulnerability has been resolved: bpf: zero extend the result of an arena 32-bit cmpxchg bpf_convert_ctx_accesses() rewrites an atomic on an arena pointer from BPF_STX | BPF_ATOMIC to BPF_STX | BPF_PROBE_ATOMIC, and it runs before bpf_opt_subreg_zext_lo32_rnd_hi32(). That pass emits an explicit zero extension for a 32-bit cmpxchg even when bpf_jit_needs_zext() is false. This is done because on some architectures 32-bit cmpxchg requires explicit zero extension for the dst register. E.g. on x86-64 'lock cmpxchg' does not change the %eax if comparison is successful, while BPF semantics declare that each operation on a 32-bit register zero extends it's upper half. is_cmpxchg_insn() matches BPF_MODE == BPF_ATOMIC only, so an arena cmpxchg misses said zero extension adjustment. This patch adjusts is_cmpxchg_insn() to match BPF_PROBE_ATOMIC alongside BPF_ATOMIC.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/1bb1be18263995b159cf5afad3a05500ba6d2a7b
• https://git.kernel.org/stable/c/6bb6ae5ab1beffda51dabf41612f501d07f10fc6
• https://git.kernel.org/stable/c/1c1b476d43a8b6c9bc04600369dd8cc39950d98e
• https://git.kernel.org/stable/c/4814ed6406f3493bd554ad046da5f7fc04833571