linux-kernel - Re: [PATCH 3/4] tools: bpftool: fix potential NULL pointer dereference in do

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Date:   Wed, 21 Nov 2018 09:18:06 -0800
From:   Y Song <ys114321@...il.com>
To:     wen.yang99@....com.cn
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Quentin Monnet <quentin.monnet@...ronome.com>,
        Jiong Wang <jiong.wang@...ronome.com>, guro@...com,
        Sandipan Das <sandipan@...ux.vnet.ibm.com>,
        John Fastabend <john.fastabend@...il.com>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, zhong.weidong@....com.cn,
        wang.yi59@....com.cn, julia.lawall@...6.fr
Subject: Re: [PATCH 3/4] tools: bpftool: fix potential NULL pointer
 dereference in do_load

On Tue, Nov 20, 2018 at 11:42 PM Wen Yang <wen.yang99@....com.cn> wrote:
>
> This patch fixes a possible null pointer dereference in
> do_load, detected by the semantic patch
> deref_null.cocci, with the following warning:
>
> ./tools/bpf/bpftool/prog.c:1021:23-25: ERROR: map_replace is NULL but dereferenced.
>
> The following code has potential null pointer references:
>  881             map_replace = reallocarray(map_replace, old_map_fds + 1,
>  882                                        sizeof(*map_replace));
>  883             if (!map_replace) {
>  884                     p_err("mem alloc failed");
>  885                     goto err_free_reuse_maps;
>  886             }
>
> ...
> 1019 err_free_reuse_maps:
> 1020         for (i = 0; i < old_map_fds; i++)
> 1021                 close(map_replace[i].fd);
> 1022         free(map_replace);

I did not see any issues here.
We have code looks like:
 867         struct map_replace *map_replace = NULL;
 868         struct bpf_program *prog = NULL, *pos;
 869         unsigned int old_map_fds = 0;
...
 948                         map_replace = reallocarray(map_replace,
old_map_fds + 1,
 949                                                    sizeof(*map_replace));
 950                         if (!map_replace) {
 951                                 p_err("mem alloc failed");
 952                                 goto err_free_reuse_maps;
 953                         }
 954                         map_replace[old_map_fds].idx = idx;
 955                         map_replace[old_map_fds].name = name;
 956                         map_replace[old_map_fds].fd = fd;
 957                         old_map_fds++;
...

old_map_fds becomes non zero if and only if map_replace is not NULL.

> Signed-off-by: Wen Yang <wen.yang99@....com.cn>
> Reviewed-by: Tan Hu <tan.hu@....com.cn>
> CC: Julia Lawall <julia.lawall@...6.fr>
> ---
>  tools/bpf/bpftool/prog.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/tools/bpf/bpftool/prog.c b/tools/bpf/bpftool/prog.c
> index 5302ee2..de42187 100644
> --- a/tools/bpf/bpftool/prog.c
> +++ b/tools/bpf/bpftool/prog.c
> @@ -1017,8 +1017,9 @@ static int do_load(int argc, char **argv)
>  err_close_obj:
>         bpf_object__close(obj);
>  err_free_reuse_maps:
> -       for (i = 0; i < old_map_fds; i++)
> -               close(map_replace[i].fd);
> +       if (map_replace)
> +               for (i = 0; i < old_map_fds; i++)
> +                       close(map_replace[i].fd);
>         free(map_replace);
>         return -1;
>  }
> --
> 2.9.5
>