Re: [RFC PATCH] usb: typec: tcpm: Ignore unsupported/unknown alternate mode requests

[Guenter Roeck <linux@roeck-us.net>]

On 7/30/19 5:07 AM, Heikki Krogerus wrote:
> On Mon, Jul 29, 2019 at 10:31:04AM -0700, Guenter Roeck wrote:
>> On Mon, Jul 29, 2019 at 05:04:57PM +0300, Heikki Krogerus wrote:
>>> Hi,
>>>
>>> On Wed, Jul 24, 2019 at 09:30:37PM -0700, Guenter Roeck wrote:
>>>> TCPM may receive PD messages associated with unknown or unsupported
>>>> alternate modes. If that happens, calls to typec_match_altmode()
>>>> will return NULL. The tcpm code does not currently take this into
>>>> account. This results in crashes.
>>>>
>>>> Unable to handle kernel NULL pointer dereference at virtual address 000001f0
>>>> pgd = 41dad9a1
>>>> [000001f0] *pgd=00000000
>>>> Internal error: Oops: 5 [#1] THUMB2
>>>> Modules linked in: tcpci tcpm
>>>> CPU: 0 PID: 2338 Comm: kworker/u2:0 Not tainted 5.1.18-sama5-armv7-r2 #6
>>>> Hardware name: Atmel SAMA5
>>>> Workqueue: 2-0050 tcpm_pd_rx_handler [tcpm]
>>>> PC is at typec_altmode_attention+0x0/0x14
>>>> LR is at tcpm_pd_rx_handler+0xa3b/0xda0 [tcpm]
>>>> ...
>>>> [<c03fbee8>] (typec_altmode_attention) from [<bf8030fb>]
>>>> 				(tcpm_pd_rx_handler+0xa3b/0xda0 [tcpm])
>>>> [<bf8030fb>] (tcpm_pd_rx_handler [tcpm]) from [<c012082b>]
>>>> 				(process_one_work+0x123/0x2a8)
>>>> [<c012082b>] (process_one_work) from [<c0120a6d>]
>>>> 				(worker_thread+0xbd/0x3b0)
>>>> [<c0120a6d>] (worker_thread) from [<c012431f>] (kthread+0xcf/0xf4)
>>>> [<c012431f>] (kthread) from [<c01010f9>] (ret_from_fork+0x11/0x38)
>>>>
>>>> Ignore PD messages if the asociated alternate mode is not supported.
>>>>
>>>> Reported-by: Douglas Gilbert <dgilbert@interlog.com>
>>>> Cc: Douglas Gilbert <dgilbert@interlog.com>
>>>> Fixes: e9576fe8e605c ("usb: typec: tcpm: Support for Alternate Modes")
>>>> Signed-off-by: Guenter Roeck <linux@roeck-us.net>
>>>> ---
>>>> Taking a stab at the problem. I don't really know if this is the correct
>>>> fix, or even if my understanding of the problem is correct, thus marking
>>>> the patch as RFC.
>>>
>>> My guess is that typec_match_altmode() is the real culprit. We can't
>>> rely on the partner mode index number when identifying the port alt
>>> mode.
>>>
>>> Douglas, can you test the attached hack instead of this patch?
>>>
>>>
>>> thanks,
>>>
>>> -- 
>>> heikki
>>
>>> diff --git a/drivers/usb/typec/tcpm/tcpm.c b/drivers/usb/typec/tcpm/tcpm.c
>>> index ec525811a9eb..033dc097ba83 100644
>>> --- a/drivers/usb/typec/tcpm/tcpm.c
>>> +++ b/drivers/usb/typec/tcpm/tcpm.c
>>> @@ -1067,12 +1067,11 @@ static int tcpm_pd_svdm(struct tcpm_port *port, const __le32 *payload, int cnt,
>>>   
>>>   	modep = &port->mode_data;
>>>   
>>> -	adev = typec_match_altmode(port->port_altmode, ALTMODE_DISCOVERY_MAX,
>>> -				   PD_VDO_VID(p[0]), PD_VDO_OPOS(p[0]));
>>> -
>>>   	pdev = typec_match_altmode(port->partner_altmode, ALTMODE_DISCOVERY_MAX,
>>>   				   PD_VDO_VID(p[0]), PD_VDO_OPOS(p[0]));
>>>   
>>> +	adev = (void *)typec_altmode_get_partner(pdev);
>>> +
>>
>> I understand that typec_altmode_get_partner() returns a const *;
>> maybe adev should be declared as const struct typec_altmode *
>> instead of using a typecast.
> 
> Yes...
> 
>> Also, typec_altmode_get_partner() can return NULL as well if pdev is NULL.
>> Is it guaranteed that typec_match_altmode() never returns NULL for pdev ?
> 
> ...and probable no. But I don't think we can receive Attention to a
> mode that hasn't been entered.
> 

If I understand correctly, the Attention was generated by a test system.
What prevents badly implemented code in the connected system from sending
such an Attention message ?

Thanks,
Guenter