linux-kernel - Re: [PATCH v1] usb: typec: altmodes/displayport: add null pointer check for sysfs nodes

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Date: Tue, 30 Jan 2024 15:08:45 -0800
From: Greg KH <gregkh@...uxfoundation.org>
To: RD Babiera <rdbabiera@...gle.com>
Cc: heikki.krogerus@...ux.intel.com, badhri@...gle.com,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH v1] usb: typec: altmodes/displayport: add null pointer
 check for sysfs nodes

On Tue, Jan 30, 2024 at 07:26:39PM +0000, RD Babiera wrote:
> The DisplayPort driver's sysfs nodes may be present to the userspace before
> typec_altmode_set_drvdata() completes in dp_altmode_probe. This means that
> a sysfs read can trigger a NULL pointer error by deferencing dp->hpd in
> hpd_show or dp->lock in pin_assignment_show, as dev_get_drvdata() returns
> NULL in those cases.
> 
> Verify dp drvdata is present in sysfs reads and writes before proceeding.

Why not populate the sysfs nodes after the assigment happens?  That's
the normal way to do this, otherwise your change looks odd because:

> 
> Fixes: 0e3bb7d6894d ("usb: typec: Add driver for DisplayPort alternate mode")
> Cc: stable@...r.kernel.org
> Signed-off-by: RD Babiera <rdbabiera@...gle.com>
> ---
>  drivers/usb/typec/altmodes/displayport.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/drivers/usb/typec/altmodes/displayport.c b/drivers/usb/typec/altmodes/displayport.c
> index 5a80776c7255..0423326219d8 100644
> --- a/drivers/usb/typec/altmodes/displayport.c
> +++ b/drivers/usb/typec/altmodes/displayport.c
> @@ -518,6 +518,9 @@ configuration_store(struct device *dev, struct device_attribute *attr,
>  	int con;
>  	int ret = 0;
>  
> +	if (!dp)
> +		return -ENODEV;
> +
>  	con = sysfs_match_string(configurations, buf);

there's nothing keeping dp from being an invalid pointer right after you
check it.  Really that might not happen, but it's hard to tell that
here.

thanks,

greg k-h