CVE-2026-90359bpf: Reject >8 byte return values on return-reading trampoline paths

Official description

In the Linux kernel, the following vulnerability has been resolved: bpf: Reject >8 byte return values on return-reading trampoline paths btf_distill_func_proto() builds the function model used for the fentry/fexit/fmod_ret/fsession trampolines and struct_ops. It has accepted a 16-byte __int128 return value since the trampoline was introduced: __get_type_size() returns the integer's type size, and the return-type check only rejected ret < 0. But the BPF trampoline preserves only 8 bytes of the return value (RAX on x86, i.e. R0). For an attach type that reads the target's return value the second half (RDX / R3) is neither saved nor restored, so a program attached to a function returning a 16-byte value corrupts the value seen by the real caller and itself observes only half of it. struct_ops trampolines have the same limitation. This affects the attach types that read the target's return value: fexit, fmod_ret and fsession (plus the _multi variants of fexit and fsession), and struct_ops. fentry/fentry_multi run before the target returns and are unaffected. Reject a >8 byte return value for these attach types in bpf_check_attach_target() and bpf_check_attach_btf_id_multi(), and for struct_ops in bpf_struct_ops_desc_init().

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/de54ba33690d2dd3c4aa686c9693505644285b0b
• https://git.kernel.org/stable/c/9a9e902bd0556052534641f7d5f868c6dc4b84ae
• https://git.kernel.org/stable/c/d36ac53fc83baf843c51b7afd6d2471f36b713d8
• https://git.kernel.org/stable/c/c48796aa6c392cde93946e5d5a9a1f1b1cf72feb