linux-kernel - Re: [PATCH 2/2] openvswitch: Fix condition check in output_userspace() by using nla

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Date:   Tue, 21 Sep 2021 13:54:43 -0700
From:   Pravin Shelar <pravin.ovn@...il.com>
To:     Jiasheng Jiang <jiasheng@...as.ac.cn>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        ovs dev <dev@...nvswitch.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] openvswitch: Fix condition check in
 output_userspace() by using nla_ok()

On Fri, Sep 17, 2021 at 1:25 AM Jiasheng Jiang <jiasheng@...as.ac.cn> wrote:
>
> Just using 'rem > 0' might be unsafe, so it's better
> to use the nla_ok() instead.
> Because we can see from the nla_next() that
> '*remaining' might be smaller than 'totlen'. And nla_ok()
> will avoid it happening.
> For example, ovs_dp_process_packet() -> ovs_execute_actions()
> -> do_execute_actions() -> output_userspace(), and attr comes
> from OVS_CB(skb)->input_vport,which restores the received packet
> from the user space.
>
> Fixes: ccb1352e76cff0524e7ccb2074826a092dd13016
> ('net: Add Open vSwitch kernel components.')
> Signed-off-by: Jiasheng Jiang <jiasheng@...as.ac.cn>

> ---
>  net/openvswitch/actions.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
> index c23537f..e8236dd 100644
> --- a/net/openvswitch/actions.c
> +++ b/net/openvswitch/actions.c
> @@ -915,8 +915,7 @@ static int output_userspace(struct datapath *dp, struct sk_buff *skb,
>         upcall.cmd = OVS_PACKET_CMD_ACTION;
>         upcall.mru = OVS_CB(skb)->mru;
>
> -       for (a = nla_data(attr), rem = nla_len(attr); rem > 0;
> -            a = nla_next(a, &rem)) {
> +       nla_for_each_nested(a, attr, rem) {
>                 switch (nla_type(a)) {
>                 case OVS_USERSPACE_ATTR_USERDATA:
>                         upcall.userdata = a;

These nl-attributes are built and verified at time of OVS flow
install, so the rest of checks in nla_ok, is not required.