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Date: Thu, 25 Apr 2024 07:30:23 +0200
From: "Christian A. Ehrhardt" <lk@c--e.de>
To: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
	Jameson Thies <jthies@google.com>, heikki.krogerus@linux.intel.com,
	linux-usb@vger.kernel.org, pmalani@chromium.org, bleung@google.com,
	andersson@kernel.org, fabrice.gasnier@foss.st.com,
	gregkh@linuxfoundation.org, hdegoede@redhat.com,
	neil.armstrong@linaro.org, rajaram.regupathy@intel.com,
	saranya.gopal@intel.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/4] usb: typec: ucsi: Fix null deref in trace
Message-ID: <Zinqbx1lofe0/Otz@cae.in-ulm.de>
References: <20240424014821.4154159-1-jthies@google.com>
 <20240424014821.4154159-2-jthies@google.com>
 <CAA8EJpq_DujhwoJ87Cg4gZ4LNdPu4i93EQ0VeKrCJPkeDj9ThQ@mail.gmail.com>
 <CANFp7mVGYhMYXdCGEJQ9GoqB-kpk4UquUWEcvqVnRFMrih+R9Q@mail.gmail.com>
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Hi,

On Wed, Apr 24, 2024 at 05:00:24PM -0700, Abhishek Pandit-Subedi wrote:
> On Tue, Apr 23, 2024 at 7:06 PM Dmitry Baryshkov
> <dmitry.baryshkov@linaro.org> wrote:
> >
> > On Wed, 24 Apr 2024 at 04:48, Jameson Thies <jthies@google.com> wrote:
> > >
> > > From: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
> > >
> > > ucsi_register_altmode checks IS_ERR on returned pointer and treats
> > > NULL as valid. This results in a null deref when
> > > trace_ucsi_register_altmode is called. Return an error from
> > > ucsi_register_displayport when it is not supported and register the
> > > altmode with typec_port_register_altmode.
> > >
> > > Reviewed-by: Jameson Thies <jthies@google.com>
> > > Signed-off-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
> > > ---
> > > Changes in V2:
> > > - Checks for error response from ucsi_register_displayport when
> > > registering DisplayPort alternate mode.
> > >
> > >  drivers/usb/typec/ucsi/ucsi.c | 3 +++
> > >  drivers/usb/typec/ucsi/ucsi.h | 2 +-
> > >  2 files changed, 4 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/usb/typec/ucsi/ucsi.c b/drivers/usb/typec/ucsi/ucsi.c
> > > index cb52e7b0a2c5c..f3b413f94fd28 100644
> > > --- a/drivers/usb/typec/ucsi/ucsi.c
> > > +++ b/drivers/usb/typec/ucsi/ucsi.c
> > > @@ -361,6 +361,9 @@ static int ucsi_register_altmode(struct ucsi_connector *con,
> > >                 switch (desc->svid) {
> > >                 case USB_TYPEC_DP_SID:
> > >                         alt = ucsi_register_displayport(con, override, i, desc);
> > > +                       if (IS_ERR(alt) && PTR_ERR(alt) == -EOPNOTSUPP)
> >
> > This makes it ignore EOPNOTSUPP if it is returned by the non-stub
> > implementation. I think the current state is actually better than the
> > implementation found in this patch. I'd suggest adding a comment to
> > ucsi_register_displayport() stub instead.
> 
> So originally on my system, I didn't have the displayport driver
> config enabled. My expectation was that the alt-mode would show up but
> would not be controllable (like all other alt-modes without drivers).
> What ends up happening is that no alt-mode shows up and trying to
> enable the trace crashes.
> 
> When the displayport support isn't there, I think it should just be
> enumerated as a normal, unsupported alt-mode.

This sounds reasonable. Thus if displayport support is not compiled in
ucsi_register_displayport() should just call typec_port_register_altmode(),
I guess.

Best regards,
Christian