On Sat, Sep 19, 2020 at 4:50 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
>
> From: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
>
> From the checks and commit messages for modify_return, it seems it was
> never the intention that it should be possible to attach a tracing program
> with expected_attach_type == BPF_MODIFY_RETURN to another BPF program.
> However, check_attach_modify_return() will only look at the function name,
> so if the target function starts with "security_", the attach will be
> allowed even for bpf2bpf attachment.
>
> Fix this oversight by also blocking the modification if a target program is
> supplied.
>
> Fixes: 18644cec714a ("bpf: Fix use-after-free in fmod_ret check")
> Fixes: 6ba43b761c41 ("bpf: Attachment verification for BPF_MODIFY_RETURN")
> Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
> ---
> kernel/bpf/verifier.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 4161b6c406bc..cb1b0f9fd770 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -11442,7 +11442,7 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
> prog->aux->attach_func_name);
> } else if (prog->expected_attach_type == BPF_MODIFY_RETURN) {
> ret = check_attach_modify_return(prog, addr);
> - if (ret)
> + if (ret || tgt_prog)
can you please do it as a separate check with a more appropriate and
meaningful message?
> verbose(env, "%s() is not modifiable\n",
> prog->aux->attach_func_name);
> }
>
