netdev - Re: [PATCH v5 5/7] can: netlink: add interface for CAN-FD Transmitter Delay Compensation (TDC)

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Date:   Wed, 18 Aug 2021 10:19:34 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Vincent MAILHOL <mailhol.vincent@...adoo.fr>
Cc:     linux-can <linux-can@...r.kernel.org>,
        Stefan Mätje <Stefan.Maetje@....eu>,
        netdev <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 5/7] can: netlink: add interface for CAN-FD
 Transmitter Delay Compensation (TDC)

On 18.08.2021 17:08:51, Vincent MAILHOL wrote:
> On Wed 18 Aug 2021 at 04:55, Marc Kleine-Budde <mkl@...gutronix.de> wrote:
> > On 15.08.2021 12:32:46, Vincent Mailhol wrote:
> > > +static int can_tdc_changelink(struct net_device *dev, const struct nlattr *nla,
> > > +                           struct netlink_ext_ack *extack)
> > > +{
> > > +     struct nlattr *tb_tdc[IFLA_CAN_TDC_MAX + 1];
> > > +     struct can_priv *priv = netdev_priv(dev);
> > > +     struct can_tdc *tdc = &priv->tdc;
> > > +     const struct can_tdc_const *tdc_const = priv->tdc_const;
> > > +     int err;
> > > +
> > > +     if (!tdc_const || !can_tdc_is_enabled(priv))
> > > +             return -EOPNOTSUPP;
> > > +
> > > +     if (dev->flags & IFF_UP)
> > > +             return -EBUSY;
> > > +
> > > +     err = nla_parse_nested(tb_tdc, IFLA_CAN_TDC_MAX, nla,
> > > +                            can_tdc_policy, extack);
> > > +     if (err)
> > > +             return err;
> > > +
> > > +     if (tb_tdc[IFLA_CAN_TDC_TDCV]) {
> > > +             u32 tdcv = nla_get_u32(tb_tdc[IFLA_CAN_TDC_TDCV]);
> > > +
> > > +             if (tdcv < tdc_const->tdcv_min || tdcv > tdc_const->tdcv_max)
> > > +                     return -EINVAL;
> > > +
> > > +             tdc->tdcv = tdcv;
> >
> > You have to assign to a temporary struct first, and set the priv->tdc
> > after complete validation, otherwise you end up with inconsistent
> > values.
> 
> Actually, copying the temporary structure to priv->tdc is not an
> atomic operation. Here, you are only reducing the window, not
> closing it.

It's not a race I'm fixing.

> 
> > > +     }
> > > +
> > > +     if (tb_tdc[IFLA_CAN_TDC_TDCO]) {
> > > +             u32 tdco = nla_get_u32(tb_tdc[IFLA_CAN_TDC_TDCO]);
> > > +
> > > +             if (tdco < tdc_const->tdco_min || tdco > tdc_const->tdco_max)
> > > +                     return -EINVAL;
> > > +
> > > +             tdc->tdco = tdco;
> > > +     }
> > > +
> > > +     if (tb_tdc[IFLA_CAN_TDC_TDCF]) {
> > > +             u32 tdcf = nla_get_u32(tb_tdc[IFLA_CAN_TDC_TDCF]);
> > > +
> > > +             if (tdcf < tdc_const->tdcf_min || tdcf > tdc_const->tdcf_max)
> > > +                     return -EINVAL;
> > > +
> > > +             tdc->tdcf = tdcf;
> > > +     }
> > > +
> > > +     return 0;
> > > +}
> >
> > To reproduce (ip pseudo-code only :D ):
> >
> > ip down
> > ip up tdc-mode manual tdco 111 tdcv 33  # 111 is out of range, 33 is valid
> > ip down
> > ip up                                   # results in tdco=0 tdcv=33 mode=manual
> 
> I do not think that this PoC would work because, thankfully, the
> netlink interface uses a mutex to prevent this issue from
> occurring.

It works, I've tested it :)

> That mutex is defined in:
> https://elixir.bootlin.com/linux/latest/source/net/core/rtnetlink.c#L68
> 
> Each time a netlink message is sent to the kernel, it would be
> dispatched by rtnetlink_rcv_msg() which will make sure to lock
> the mutex before doing so:
> https://elixir.bootlin.com/linux/latest/source/net/core/rtnetlink.c#L5551
> 
> A funny note is that because the mutex is global, if you run two
> ip command in a row:
> 
> | ip link set can0 type can bitrate 500000
> | ip link set can1 up
> 
> the second one will wait for the first one to finish even if it
> is on a different network device.
> 
> To conclude, I do not think this needs to be fixed.

It's not a race. Consider this command:

| ip up tdc-mode manual tdco 111 tdcv 33  # 111 is out of range, 33 is valid

tdcv is checked first and valid, then it's assigned to the priv->tdc.
tdco is checked second and invalid, then can_tdc_changelink() returns -EINVAL.

tdc ends up being half set :(

So the setting of tdc is inconsistent and when you do a "ip down" "ip
up" then it results in a tdco=0 tdcv=33 mode=manual.

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |

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