Re: [Qemu-devel] [PATCH] net: avoid to use variable length array in net_client_init()

[Markus Armbruster <armbru@redhat.com>]

Stefano Garzarella <sgarzare@redhat.com> writes:

> net_client_init() uses a variable length array to store the prefix
> of 'ipv6-net' parameter (e.g. if ipv6-net=fec0::0/64, the prefix
> is 'fec0::0').
> Since the IPv6 prefix can be at most as long as an IPv6 address,
> we can use an array with fixed size equals to INET6_ADDRSTRLEN.
>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
>  net/net.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/net.c b/net/net.c
> index f3a3c5444c..2e5f27e121 100644
> --- a/net/net.c
> +++ b/net/net.c
> @@ -1118,7 +1118,7 @@ static int net_client_init(QemuOpts *opts, bool is_netdev, Error **errp)
>          const char *ip6_net = qemu_opt_get(opts, "ipv6-net");
>  
>          if (ip6_net) {
> -            char buf[strlen(ip6_net) + 1];
> +            char buf[INET6_ADDRSTRLEN];
>  
>              if (get_str_sep(buf, sizeof(buf), &ip6_net, '/') < 0) {
>                  /* Default 64bit prefix length.  */

Hmm.

Parameter "ipv6-net" is of the form ADDRESS[/PREFIX-SIZE].  If
/PREFIX-SIZE is present, get_str_sep() copies the ADDRESS part to buf[].

However, nothing stops the user from passing in an ADDRESS longer than
INET6_ADDRSTRLEN, say by adding a enough leading zeros.  get_str_sep()
will then silently truncate ADDRESS.

Suggest to avoid get_str_sep() like this (not even compile-tested):

        if (ip6_net) {
            char *slashp = strchr(ip6_net, '/');

            if (!slashp) {
                /* Default 64bit prefix length.  */
                qemu_opt_set(opts, "ipv6-prefix", ip6_net, &error_abort);
                qemu_opt_set_number(opts, "ipv6-prefixlen", 64, &error_abort);
            } else {
                /* User-specified prefix length.  */
                unsigned long len;
                int err;
                char *addr = g_strndup(ip6_net, slashp - ip6_net);

                qemu_opt_set(opts, "ipv6-prefix", addr, &error_abort);
                g_free(addr);
                err = qemu_strtoul(slashp + 1, NULL, 10, &len);
                if (err) {
                    error_setg(errp, QERR_INVALID_PARAMETER_VALUE,
                              "ipv6-prefix", "a number");
                } else {
                    qemu_opt_set_number(opts, "ipv6-prefixlen", len,
                                        &error_abort);
                }
            }
            qemu_opt_unset(opts, "ipv6-net");
        }
    }

I'd be tempted to clean up further; de-duplicate the qemu_opt_set() and
qemu_opt_set_number().

There's just one more use of get_str_sep(), in parse_host_port(), and it
looks just as prone to silent truncation.