linux-kernel - Re: [RESEND PATCH v2] can: netlink: prevent incoherent can configuration in case of early return

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Date:   Mon, 6 Sep 2021 10:18:05 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Vincent Mailhol <mailhol.vincent@...adoo.fr>
Cc:     linux-can@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v2] can: netlink: prevent incoherent can
 configuration in case of early return

On 03.09.2021 16:17:04, Vincent Mailhol 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).

I don't like the optimization of using a static priv. If it's too big to
be allocated on the stack, allocate it on the heap, i.e. using
kmemdup()/kfree().

> 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.
> 
> Finally, function can_calc_tdco() was retrieving can_priv from the
> net_device and directly modifying it. We changed the prototype so that
> it instead writes its changes into our temporary priv variable.

Is it possible to split this into a separate patch, so that the part
without the tdco can be backported more easily to older kernels not
having tdco? The patch fixing the tdco would be the 2nd patch...

> Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
> ---
> Resending because I got no answers on:
> https://lore.kernel.org/linux-can/20210823024750.702542-1-mailhol.vincent@wanadoo.fr/T/#u
> (I guess everyone bas busy with the upcoming merge window)

Busy yes, but not with the merge window :)

> I am not sure whether or not this needs a "Fixes" tag. Just in case,
> there it is:
> 
> Fixes: 9859ccd2c8be ("can: introduce the data bitrate configuration for CAN FD")

...if it's possible to split this patch into 2 parts, add individual
fixes tags to them.

regards,
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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