On Tue, Sep 22, 2020 at 11:39 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>
> ---
Acked-by: Andrii Nakryiko <andriin@xxxxxx>
> kernel/bpf/verifier.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 15ab889b0a3f..797e2b0d8bc2 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -11471,6 +11471,11 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
> verbose(env, "%s is not sleepable\n",
> prog->aux->attach_func_name);
> } else if (prog->expected_attach_type == BPF_MODIFY_RETURN) {
> + if (tgt_prog) {
> + verbose(env, "can't modify return codes of BPF programs\n");
> + ret = -EINVAL;
> + goto out;
> + }
> ret = check_attach_modify_return(prog, addr);
> if (ret)
> verbose(env, "%s() is not modifiable\n",
>
