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

Summary

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().

Timeline

1. Sep 17, 2026, 4:09 PM UTCCVE published
2. Oct 21, 2026, 12:51 PM UTCRecord last updated

Affected versions

• Linux Linux

  From fec56f5890d93fc2ed74166c397dc186b1c25951 · fixed in de54ba33690d2dd3c4aa686c9693505644285b0b; From fec56f5890d93fc2ed74166c397dc186b1c25951 · fixed in 9a9e902bd0556052534641f7d5f868c6dc4b84ae; From fec56f5890d93fc2ed74166c397dc186b1c25951 · fixed in d36ac53fc83baf843c51b7afd6d2471f36b713d8; From fec56f5890d93fc2ed74166c397dc186b1c25951 · fixed in c48796aa6c392cde93946e5d5a9a1f1b1cf72feb

• Linux Linux

  5.5

• KernelLinux

  From 5.5.0 · fixed in 6.12.114; From 6.13.0 · fixed in 6.18.57; From 6.19.0 · fixed in 7.2.6

References 4