linux-kernel - Re: [PATCH v2] usb: core: Prevent null pointer dereference in update_port_device

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Date: Mon, 8 Jan 2024 21:06:12 +0530
From: Udipto Goswami <quic_ugoswami@...cinc.com>
To: Alan Stern <stern@...land.harvard.edu>
CC: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Krishna Kurapati
	<quic_kriskura@...cinc.com>,
        <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH v2] usb: core: Prevent null pointer dereference in
 update_port_device_state

On 1/8/2024 8:56 PM, Alan Stern wrote:
> On Mon, Jan 08, 2024 at 06:37:06PM +0530, Udipto Goswami wrote:
>> Currently,the function update_port_device_state gets the usb_hub from
>> udev->parent by calling usb_hub_to_struct_hub.
>> However, in case the actconfig or the maxchild is 0, the usb_hub would
>> be NULL and upon further accessing to get port_dev would result in null
>> pointer dereference.
>>
>> Fix this by introducing an if check after the usb_hub is populated.
>>
>> Fixes: 83cb2604f641 ("usb: core: add sysfs entry for usb device state")
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Udipto Goswami <quic_ugoswami@...cinc.com>
>> ---
>> v2: Introduced comment for the if check & CC'ed stable.
>>
>>   drivers/usb/core/hub.c | 15 ++++++++++++---
>>   1 file changed, 12 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
>> index ffd7c99e24a3..d40b5500f95b 100644
>> --- a/drivers/usb/core/hub.c
>> +++ b/drivers/usb/core/hub.c
>> @@ -2053,9 +2053,18 @@ static void update_port_device_state(struct usb_device *udev)
>>   
>>   	if (udev->parent) {
>>   		hub = usb_hub_to_struct_hub(udev->parent);
>> -		port_dev = hub->ports[udev->portnum - 1];
>> -		WRITE_ONCE(port_dev->state, udev->state);
>> -		sysfs_notify_dirent(port_dev->state_kn);
>> +
>> +		/*
>> +		 * usb_hub_to_struct_hub() if returns NULL can
>> +		 * potentially cause NULL pointer dereference upon further
>> +		 * access.
>> +		 * Avoid this with an if check.
>> +		 */
> 
> This is not what I meant.  It's perfectly obvious that if
> usb_hub_to_struct_hub() returns NULL then there will be a NULL-pointer
> dereference.  You don't need to explain that to anybody.
> 
> Instead, you need to explain why it is _possible_ for
> usb_hub_to_struct_hub() to return NULL.  The reason is because the
> lvstest driver messes around with usbcore internals without telling the
> hub driver, so hub will be NULL in cases where udev was created by
> lvstest.

aah okay, Thanks for the clarification. I'll re-write the comment 
mentioning this.

Thanks,
-Udipto