On Wed 18 Aug 2021 at 04:55, Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx> 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.
> > + }
> > +
> > + 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.
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.
Yours sincerely,
Vincent
