netdev - Re: [PATCH bpf 1/2] xdp: check prog type before updating BPF link

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Date:   Fri, 7 Jan 2022 13:17:39 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        syzbot+983941aa85af6ded1fd9@...kaller.appspotmail.com,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf 1/2] xdp: check prog type before updating BPF link

On Fri, Jan 7, 2022 at 10:31 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> The bpf_xdp_link_update() function didn't check the program type before
> updating the program, which made it possible to install any program type as
> an XDP program, which is obviously not good. Syzbot managed to trigger this
> by swapping in an LWT program on the XDP hook which would crash in a helper
> call.
>
> Fix this by adding a check and bailing out if the types don't match.
>
> Fixes: 026a4c28e1db ("bpf, xdp: Implement LINK_UPDATE for BPF XDP link")
> Reported-by: syzbot+983941aa85af6ded1fd9@...kaller.appspotmail.com
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---

The fix looks good to me, thanks. I'd love it if this was done
generically in link_update, but each link type has its own locking
schema for link->prog, so I didn't figure out a way to do this in a
centralized way.

Acked-by: Andrii Nakryiko <andrii@...nel.org>

>  net/core/dev.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index c4708e2487fb..2078d04c6482 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -9656,6 +9656,12 @@ static int bpf_xdp_link_update(struct bpf_link *link, struct bpf_prog *new_prog,
>                 goto out_unlock;
>         }
>         old_prog = link->prog;
> +       if (old_prog->type != new_prog->type ||
> +           old_prog->expected_attach_type != new_prog->expected_attach_type) {
> +               err = -EINVAL;
> +               goto out_unlock;
> +       }
> +
>         if (old_prog == new_prog) {
>                 /* no-op, don't disturb drivers */
>                 bpf_prog_put(new_prog);
> --
> 2.34.1
>