bpf: Reject writes through untrusted BTF pointers

In the Linux kernel, the following vulnerability has been resolved: bpf: Reject writes through untrusted BTF pointers check_ptr_to_btf_access() lets program-type btf_struct_access callbacks validate writes before the default BTF access path rejects non-read accesses. That bypasses the read-only policy for untrusted BTF pointers created by helpers such as bpf_rdonly_cast(). Reject non-read accesses through PTR_UNTRUSTED BTF pointers at the common entry point, before the callback branch to handle all cases.

Where it's fixed