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

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Date:   Wed, 21 Nov 2018 09:30:44 -0800
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Y Song <ys114321@...il.com>
Cc:     wen.yang99@....com.cn, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        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 Wed, 21 Nov 2018 09:18:06 -0800, Y Song wrote:
> 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.

Note that this is a realloc in a loop, and map_replace will become NULL
again if realloc fails.  We should check the return value of realloc()
without loosing the pointer to the old value.  No?

> > 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
> >