linux-kernel - Re: [PATCH] usb: typec: fix potential NULL dereference

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Date:   Tue, 18 Apr 2023 09:56:51 +0300
From:   Maxim Korotkov <korotkov.maxim.s@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        lvc-project@...uxtesting.org
Subject: Re: [PATCH] usb: typec: fix potential NULL dereference

On 18.04.2023 09:16, Greg Kroah-Hartman wrote:
> On Mon, Apr 17, 2023 at 10:50:03PM +0300, Maxim Korotkov wrote:
>> The pointer 'adev' was being dereferenced before being checked for NULL
>> in the 'type_alt mode_enter()' and 'type_alt mode_exit()' functions.
>> Although this is a hypothetical issue, it's better to move the pointer
>> assignment after the NULL check to avoid any potential problems.
>>
>> Found by Linux Verification Center with Svace static analyzer.
>>
>> Fixes: 8a37d87d72f0 ("usb: typec: Bus type for alternate modes")
>> Signed-off-by: Maxim Korotkov <korotkov.maxim.s@...il.com>
>> ---
>>   drivers/usb/typec/bus.c | 13 +++++++++----
>>   1 file changed, 9 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/usb/typec/bus.c b/drivers/usb/typec/bus.c
>> index 098f0efaa58d..ae0aca8f33db 100644
>> --- a/drivers/usb/typec/bus.c
>> +++ b/drivers/usb/typec/bus.c
>> @@ -125,13 +125,16 @@ EXPORT_SYMBOL_GPL(typec_altmode_notify);
>>    */
>>   int typec_altmode_enter(struct typec_altmode *adev, u32 *vdo)
>>   {
>> -	struct altmode *partner = to_altmode(adev)->partner;
>> -	struct typec_altmode *pdev = &partner->adev;
>> +	struct altmode *partner;
>> +	struct typec_altmode *pdev;
>>   	int ret;
>>   
>>   	if (!adev || adev->active)
>>   		return 0;
>>   
>> +	partner = to_altmode(adev)->partner;
>> +	pdev = &partner->adev;
> 
> As you point out, the original code is still fine here, we check before
> we actually use these values.
> 
> Also, can adev every actually be NULL?  In looking at the code paths, I
> can't see how that could happen.
> 
> thanks,
> 
> greg k-h

I agree that the adev will most likely never be NULL, but usually this 
pointer is checked before usage (for example in typec_altmode_notify() 
or typec_altmode_vdm()). It is a little odd that in these functions it 
utilized before check. Is it just extra check that can be removed?

best regards, Max