Re: [PATCH v3 1/2] can: netlink: prevent incoherent can configuration in case of early return

[Oliver Hartkopp <socketcan@hartkopp.net>]

- nextdev ML
- linux-kernel ML

Hi Vincent,

On 07.09.21 14:51, Vincent MAILHOL wrote:
> On Tue. 7 Sep. 2021 at 01:03, Vincent Mailhol
> <mailhol.vincent@wanadoo.fr> wrote:
>> struct can_priv has a set of flags (can_priv::ctrlmode) which are
>> correlated with the other fields of the structure. In
>> can_changelink(), those flags are set first and copied to can_priv. If
>> the function has to return early, for example due to an out of range
>> value provided by the user, then the global configuration might become
>> incoherent.
>>
>> Example: the user provides an out of range dbitrate (e.g. 20
>> Mbps). The command fails (-EINVAL), however the FD flag was already
>> set resulting in a configuration where FD is on but the databittiming
>> parameters are empty.

When the ip configuration fails you get an error code. And you 
*typically* do it again to fix your wrong command line parameters.

¯\_(ツ)_/¯

If not the attempt to set the CAN interface to 'up' will fail (as the 
last line of defense).

The code with all the sanity checks is already pretty complex IMO.

I wonder if this effort is worth it.

Best regards,
Oliver



>>
>> * Illustration of above example *
>>
>> | $ ip link set can0 type can bitrate 500000 dbitrate 20000000 fd on
>> | RTNETLINK answers: Invalid argument
>> | $ ip --details link show can0
>> | 1: can0: <NOARP,ECHO> mtu 72 qdisc noop state DOWN mode DEFAULT group default qlen 10
>> |     link/can  promiscuity 0 minmtu 0 maxmtu 0
>> |     can <FD> state STOPPED restart-ms 0
>>             ^^ FD flag is set without any of the databittiming parameters...
>> |         bitrate 500000 sample-point 0.875
>> |         tq 12 prop-seg 69 phase-seg1 70 phase-seg2 20 sjw 1
>> |         ES582.1/ES584.1: tseg1 2..256 tseg2 2..128 sjw 1..128 brp 1..512 brp-inc 1
>> |         ES582.1/ES584.1: dtseg1 2..32 dtseg2 1..16 dsjw 1..8 dbrp 1..32 dbrp-inc 1
>> |         clock 80000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535
>>
>> To prevent this from happening, we do a local copy of can_priv, work
>> on it, an copy it at the very end of the function (i.e. only if all
>> previous checks succeeded).
>>
>> Once this done, there is no more need to have a temporary variable for
>> a specific parameter. As such, the bittiming and data bittiming (bt
>> and dbt) are directly written to the temporary priv variable.
>>
>>
>> N.B. The temporary can_priv is too big to be allocated on the stack
>> because, on x86_64 sizeof(struct can_priv) is 448 and:
>>
>> | $ objdump -d drivers/net/can/dev/netlink.o | ./scripts/checkstack.pl
>> | 0x00000000000002100 can_changelink []:            1200
>>
>>
>> Fixes: 9859ccd2c8be ("can: introduce the data bitrate configuration for CAN FD")
>> Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
>> ---
>>   drivers/net/can/dev/netlink.c | 32 ++++++++++++++++++--------------
>>   1 file changed, 18 insertions(+), 14 deletions(-)
>>
>> diff --git a/drivers/net/can/dev/netlink.c b/drivers/net/can/dev/netlink.c
>> index 80425636049d..21b76ca8cb22 100644
>> --- a/drivers/net/can/dev/netlink.c
>> +++ b/drivers/net/can/dev/netlink.c
>> @@ -58,14 +58,19 @@ static int can_changelink(struct net_device *dev, struct nlattr *tb[],
>>                            struct nlattr *data[],
>>                            struct netlink_ext_ack *extack)
>>   {
>> -       struct can_priv *priv = netdev_priv(dev);
>> +       /* Work on a local copy of priv to prevent inconsistent value
>> +        * in case of early return.
>> +        */
>> +       static struct can_priv *priv;
>>          int err;
>>
>>          /* We need synchronization with dev->stop() */
>>          ASSERT_RTNL();
>>
>> +       priv = kmemdup(netdev_priv(dev), sizeof(*priv), GFP_KERNEL);
>> +
>>          if (data[IFLA_CAN_BITTIMING]) {
>> -               struct can_bittiming bt;
>> +               struct can_bittiming *bt = &priv->bittiming;
>>
>>                  /* Do not allow changing bittiming while running */
>>                  if (dev->flags & IFF_UP)
>> @@ -79,22 +84,20 @@ static int can_changelink(struct net_device *dev, struct nlattr *tb[],
>>                  if (!priv->bittiming_const && !priv->do_set_bittiming)
>>                          return -EOPNOTSUPP;
>>
>> -               memcpy(&bt, nla_data(data[IFLA_CAN_BITTIMING]), sizeof(bt));
>> -               err = can_get_bittiming(dev, &bt,
>> +               memcpy(bt, nla_data(data[IFLA_CAN_BITTIMING]), sizeof(*bt));
>> +               err = can_get_bittiming(dev, bt,
>>                                          priv->bittiming_const,
>>                                          priv->bitrate_const,
>>                                          priv->bitrate_const_cnt);
>>                  if (err)
>>                          return err;
>>
>> -               if (priv->bitrate_max && bt.bitrate > priv->bitrate_max) {
>> +               if (priv->bitrate_max && bt->bitrate > priv->bitrate_max) {
>>                          netdev_err(dev, "arbitration bitrate surpasses transceiver capabilities of %d bps\n",
>>                                     priv->bitrate_max);
>>                          return -EINVAL;
>>                  }
>>
>> -               memcpy(&priv->bittiming, &bt, sizeof(bt));
>> -
>>                  if (priv->do_set_bittiming) {
>>                          /* Finally, set the bit-timing registers */
>>                          err = priv->do_set_bittiming(dev);
> 
> Actually, there is a big issue with my approach: the
> do_set_bittiming() and some other callback functions need to
> access priv. However, the changes being in the temporary
> variable, these will not be visible by the device.
> 
> I will rework all that a little bit more before sending v4.
> 
> 
> Yours sincerely,
> Vincent Mailhol
>