Re: WARNING in usb_free_urb

[Hans Verkuil <hverkuil+cisco@kernel.org>]

There are a lot of lifetime issues in em28xx.

This patch series should fix them:

https://patchwork.linuxtv.org/project/linux-media/list/?series=26968

If you can, then please test with this series and see if this issue
still appears.

Regards,

	Hans

On 29/06/2026 09:31, Vlastimil Babka (SUSE) wrote:
> On 6/29/26 09:20, Michal Pecio wrote:
>> On Mon, 29 Jun 2026 08:27:48 +0200, Vlastimil Babka (SUSE) wrote:
>>> On 6/26/26 23:27, sanan.hasanou@gmail.com wrote:
>>>> Good day, dear maintainers,
>>>>
>>>> We found a bug using a modified version of syzkaller.  
>>>
>>> Subject says "usb_free_urb" but you only CC'd slab maintainers, where slab
>>> slab is most likely a victim here of e.g. double kfree() or a kfree() of
>>> otherwise broken pointer.
>>>
>>> Ccing USB and EM28XX maintainers. But they can feel free to ignore this per
>>> the next point.
>>>
>>>> Kernel Branch: 7.0-rc1  
>>>
>>> Why use such a version for fuzzing? rc1 will have many bugs that are
>>> already fixed in 7.0 final. And it's not even latest, 7.1 was
>>> released 2 weeks ago too.
>>
>> To be fair, em28xx had no changes since 2024 until 7.1-rc1, so the bug
>> must be present in various stable releases and likely in mainline too.
> 
> OK I didn't check that, but in general my comment stands.
> 
>>>> WARNING: mm/slub.c:6352 at free_large_kmalloc+0xb3/0x160 mm/slub.c:6352, CPU#1: kworker/1:4/12317  
>>>
>>> A kfree() was attempted on a pointer that's neither from a slab page nor a
>>> large kmalloc page. Might be double free or corrupted.
>>>
>>>> Call Trace:
>>>>  <TASK>
>>>>  kfree+0xae/0x630 mm/slub.c:6437
>>>>  urb_destroy drivers/usb/core/urb.c:25 [inline]  
>>>
>>> static void urb_destroy(struct kref *kref)
>>> {
>>>         struct urb *urb = to_urb(kref);
>>>
>>>         if (urb->transfer_flags & URB_FREE_BUFFER)
>>>                 kfree(urb->transfer_buffer);  <--- this one
>>>
>>>         kfree(urb);
>>> }
>>>
>>>>  kref_put include/linux/kref.h:65 [inline]
>>>>  usb_free_urb+0xd1/0x120 drivers/usb/core/urb.c:96  
>>>
>>> USB layer itself is likely also not the root cause.
>>>
>>>>  em28xx_uninit_usb_xfer+0x165/0x310 drivers/media/usb/em28xx/em28xx-core.c:833
>>>>  em28xx_alloc_urbs+0xf2a/0x1130 drivers/media/usb/em28xx/em28xx-core.c:-1
>>>>  em28xx_dvb_init+0x2b0/0x4a20 drivers/media/usb/em28xx/em28xx-dvb.c:-1
>>>>  em28xx_init_extension+0x121/0x1d0 drivers/media/usb/em28xx/em28xx-core.c:1117  
>>>
>>> So it might be this driver doing something wrong?
>>
>> Yes, it is. 
> 
> Cool :)
> 
>>         /* allocate urbs and transfer buffers */
>>         for (i = 0; i < usb_bufs->num_bufs; i++) {
>>                 urb = usb_alloc_urb(usb_bufs->num_packets, GFP_KERNEL);
>>                 if (!urb) {
>>                         em28xx_uninit_usb_xfer(dev, mode);
>>                         return -ENOMEM;
>>                 }
>>                 usb_bufs->urb[i] = urb;
>>
>>                 usb_bufs->buf[i] = kzalloc(sb_size, GFP_KERNEL);
>>                 if (!usb_bufs->buf[i]) {
>>                         for (i--; i >= 0; i--)
>>                                 kfree(usb_bufs->buf[i]);
>>
>>                         em28xx_uninit_usb_xfer(dev, mode);
>>                         return -ENOMEM;
>>                 }
>>
>>                 urb->transfer_flags = URB_FREE_BUFFER;
>>
>> If buf[i] allocation fails, all previous buffers are freed and then all
>> previous URBs are destroyed. But they already have the URB_FREE_BUFFER
>> flag set, which causes a double free as shown above.
>>
>> The free(buf[i]) loop should simply be removed. It was mistakenly added
>> by d571b592c6206, then a26efd1961a18 recognized the double free but
>> attempted to fix it only by changing the order of freeing. Sent from
>> .edu domain, so probably an automatic static analyzer fix...
>>
>> Regards,
>> Michal
> 
>