linux-kernel - Re: [PATCH v1] usb: typec: class: fix typec_altmode_put

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Message-ID: <ZV8g0nkZSLI8YXyd@kuha.fi.intel.com>
Date:   Thu, 23 Nov 2023 11:52:18 +0200
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     RD Babiera <rdbabiera@...gle.com>
Cc:     gregkh@...uxfoundation.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, badhri@...gle.com,
        stable@...r.kernel.org
Subject: Re: [PATCH v1] usb: typec: class: fix typec_altmode_put_partner to
 put plugs

On Tue, Nov 21, 2023 at 08:39:55PM +0000, RD Babiera wrote:
> When releasing an Alt Mode, typec_altmode_release called by a plug device
> will not release the plug Alt Mode, meaning that a port will hold a
> reference to a plug Alt Mode even if the port partner is unregistered.
> As a result, typec_altmode_get_plug() can return an old plug altmode.
> 
> Currently, typec_altmode_put_partner does not raise issues
> when unregistering a partner altmode. Looking at the current
> implementation:
> 
> > static void typec_altmode_put_partner(struct altmode *altmode)
> > {
> >	struct altmode *partner = altmode->partner;
> 
> When called by the partner Alt Mode, then partner evaluates to the port's
> Alt Mode. When called by the plug Alt Mode, this also evaluates to the
> port's Alt Mode.
> 
> >	struct typec_altmode *adev;
> >
> >	if (!partner)
> >		return;
> >
> >	adev = &partner->adev;
> 
> This always evaluates to the port's typec_altmode
> 
> >	if (is_typec_plug(adev->dev.parent)) {
> >		struct typec_plug *plug = to_typec_plug(adev->dev.parent);
> >
> >		partner->plug[plug->index] = NULL;
> 
> If the routine is called to put the plug's Alt mode and altmode refers to
> the plug, then adev referring to the port can never be a typec_plug. If
> altmode refers to the port, adev will always refer to the port partner,
> which runs the block below.
> 
> >	} else {
> >		partner->partner = NULL;
> >	}
> >	put_device(&adev->dev);
> > }
> 
> When calling typec_altmode_set_partner, a registration always calls
> get_device() on the port partner or the plug being registered, therefore
> typec_altmode_put_partner should put_device() the same device. By changing
> adev to altmode->adev, we make sure to put the correct device and properly
> unregister plugs. The reason port partners are always properly
> unregistered is because even when adev refers to the port, the port
> partner gets nullified in the else block. The port device currently gets
> put().
> 
> Fixes: 8a37d87d72f0 ("usb: typec: Bus type for alternate modes")
> Cc: stable@...r.kernel.org
> Signed-off-by: RD Babiera <rdbabiera@...gle.com>
> ---
>  drivers/usb/typec/class.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/typec/class.c b/drivers/usb/typec/class.c
> index 2e0451bd336e..803be1943445 100644
> --- a/drivers/usb/typec/class.c
> +++ b/drivers/usb/typec/class.c
> @@ -267,7 +267,7 @@ static void typec_altmode_put_partner(struct altmode *altmode)
>  	if (!partner)
>  		return;
>  
> -	adev = &partner->adev;
> +	adev = &altmode->adev;
>  
>  	if (is_typec_plug(adev->dev.parent)) {
>  		struct typec_plug *plug = to_typec_plug(adev->dev.parent);

Sorry, I may have missed something, but do we need to call this
function with ports at all?

static void typec_altmode_release(struct device *dev)
{
        struct altmode *alt = to_altmode(to_typec_altmode(dev));
 
-       typec_altmode_put_partner(alt);
+       if (!is_typec_port(dev->parent))
+               typec_altmode_put_partner(alt);
 
        altmode_id_remove(alt->adev.dev.parent, alt->id);
        kfree(alt);
        ...

thanks,

-- 
heikki