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From:   Vincent MAILHOL <mailhol.vincent@wanadoo.fr>
Date:   Tue, 7 Sep 2021 11:05:42 +0900
Message-ID: <CAMZ6RqK4DFOqpvarZa+ee1h0Z1GCpMMYFLzLQsbBjhKRTkgNdA@mail.gmail.com>
Subject: Re: [PATCH v3 1/2] can: netlink: prevent incoherent can configuration
 in case of early return
To:     Marc Kleine-Budde <mkl@pengutronix.de>,
        linux-can <linux-can@vger.kernel.org>
Cc:     netdev <netdev@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
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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.
>
> * 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);

Arg... I forgot to check the return value.
+       if (!priv)
+               return -ENOMEM;

I will send a v4, sorry for the noise.


Yours sincerely,
Vincent