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

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Date:   Thu, 08 Apr 2021 13:35:24 +0200
From:   Oliver Neukum <oneukum@...e.com>
To:     Johan Hovold <johan@...nel.org>
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

Am Donnerstag, den 08.04.2021, 11:42 +0200 schrieb Johan Hovold:
> 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.)

OK, thanks for clarification. Yes the fix makes sense.

	Regards
		Oliver