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Date:   Tue, 22 Sep 2020 11:52:16 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        KP Singh <kpsingh@...omium.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v7 01/10] bpf: disallow attaching modify_return
 tracing functions to other BPF programs

Andrii Nakryiko <andrii.nakryiko@...il.com> writes:

> On Sat, Sep 19, 2020 at 4:50 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>>
>> From: Toke Høiland-Jørgensen <toke@...hat.com>
>>
>> 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@...hat.com>
>> ---
>>  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?

Heh, okay, maybe I was being a bit too lazy here ;)

-Toke

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