linux-kernel - Re: [PATCH] libbpf: potential NULL dereference in usdt_manager_attach

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Date:   Thu, 7 Apr 2022 12:04:52 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Haowen Bai <baihaowen@...zu.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] libbpf: potential NULL dereference in usdt_manager_attach_usdt()

On Wed, Apr 6, 2022 at 7:38 PM Haowen Bai <baihaowen@...zu.com> wrote:
>
> link could be null but still dereference bpf_link__destroy(&link->link)
> and it will lead to a null pointer access.
>
> Signed-off-by: Haowen Bai <baihaowen@...zu.com>
> ---
>  tools/lib/bpf/usdt.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
> index 1bce2eab5e89..b02ebc4ba57c 100644
> --- a/tools/lib/bpf/usdt.c
> +++ b/tools/lib/bpf/usdt.c
> @@ -996,7 +996,7 @@ struct bpf_link *usdt_manager_attach_usdt(struct usdt_manager *man, const struct
>         link = calloc(1, sizeof(*link));
>         if (!link) {
>                 err = -ENOMEM;
> -               goto err_out;
> +               goto link_err;

this is not a complete fix because there are two more similar goto
err_out; above which you didn't fix. I think better fix is to just add
if (link) check before bpf_link__destroy(), which is what I did
locally when applying.


>         }
>
>         link->usdt_man = man;
> @@ -1072,7 +1072,7 @@ struct bpf_link *usdt_manager_attach_usdt(struct usdt_manager *man, const struct
>
>  err_out:
>         bpf_link__destroy(&link->link);
> -
> +link_err:
>         free(targets);
>         hashmap__free(specs_hash);
>         if (elf)
> --
> 2.7.4
>