linux-kernel - Re: [PATCH 2/3] USB: cdc-acm: fix unprivileged TIOCCSERIAL

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Message-ID: <YG7P8EJ1obdM01J8@hovoldconsulting.com>
Date:   Thu, 8 Apr 2021 11:42:08 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Oliver Neukum <oneukum@...e.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] USB: cdc-acm: fix unprivileged TIOCCSERIAL

On Thu, Apr 08, 2021 at 09:48:38AM +0200, Oliver Neukum wrote:
> Am Mittwoch, den 07.04.2021, 12:28 +0200 schrieb Johan Hovold:
> > TIOCSSERIAL is a horrid, underspecified, legacy interface which for most
> > serial devices is only useful for setting the close_delay and
> > closing_wait parameters.
> > 
> > A non-privileged user has only ever been able to set the since long
> > deprecated ASYNC_SPD flags and trying to change any other *supported*
> > feature should result in -EPERM being returned. Setting the current
> > values for any supported features should return success.
> > 
> > Fix the cdc-acm implementation which instead indicated that the
> > TIOCSSERIAL ioctl was not even implemented when a non-privileged user
> > set the current values.
> 
> Hi,
> 
> the idea here was that you are setting something else, if you are
> not changing a parameter that can be changed. That conclusion is
> dubious, but at the same time, this implementation can change
> only these two parameters. So can the test really be dropped
> as opposed to be modified?

The de-facto standard for how to handle change requests for
non-supported features (e.g. changing the I/O port or IRQ) is to simply
ignore them and return 0.

For most (non-legacy) serial devices the only relevant parameters are
close_delay and closing_wait. And as we need to return -EPERM when a
non-privileged user tries to change these, we cannot drop the test.

(And returning -EOPNOTSUPP was never correct as the ioctl is indeed
supported.)

Johan