Re: [REGRESSION] usb: gadget: u_ether NULL deref in eth_stop after gether_detach_gadget

[Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>]

On 27.05.26 г. 11:03 ч., Greg Kroah-Hartman wrote:
> On Tue, May 26, 2026 at 08:56:15AM +0300, Ivaylo Dimitrov wrote:
>> on linux 6.18.31 I am seeing a NULL pointer dereference during RNDIS gadget
>> teardown.
> 
> Does this also happen on the latest 7.1-rc release?
> 

I didn't test this on current -rc (this is a mobile phone and not 
particularly easy to run latest Linux on), however this looks more like 
an object lifetime contract issue than something specific to 6.18.y.

Also, the suspected patch was backported to 6.18.y together with 
e002e92e88e12457373ed096b18716d97e7bbb20 ("usb: gadget: u_ether: Fix 
NULL pointer deref in eth_get_drvinfo"), which fixes a similar issue, so 
I strongly suspect the issue exists in current -rc as well.

>> I suspect the reason is commit:
>>
>>    usb: gadget: f_ncm: Fix net_device lifecycle with device_move
> 
> Do you have a proposed fix for this issue?
> 

Not really, as checking whether dev->gadget is NULL before calling DBG() 
looks more like a workaround than a proper fix to me. I also don't know 
the subsystem well enough to judge whether switching to netdev_dbg() 
instead of DBG() would be appropriate here, or whether that would defeat 
the purpose of the existing debug messages (or cause confusion by mixing 
gadget and netdev based logging).

I could use some maintainer advice on what they think the proper fix 
should look like, and then I'll try to put together a fix.

Thanks,
Ivo