Re: WARNING in usb_free_urb

Re: WARNING in usb_free_urb

[Vlastimil Babka (SUSE)]

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.

> Kernel Config: <https://drive.google.com/open?id=1zJHAs5GUroGFBkxAlzfDaWAd_NVPZTfJ>
> Unfortunately, we don't have any reproducer for this bug yet.
> Thank you!
> 
> Best regards,
> Sanan Hasanov
> 
> 179683 pages reserved
> 0 pages cma reserved
> Memory cgroup min protection 0kB -- low protection 0kB
> ------------[ cut here ]------------
> !PageLargeKmalloc(page)
> 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.

> Modules linked in:
> CPU: 1 UID: 0 PID: 12317 Comm: kworker/1:4 Tainted: G             L      7.0.0-rc1 #1 PREEMPT(full) 
> Tainted: [L]=SOFTLOCKUP
> Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> Workqueue: events request_module_async
> RIP: 0010:free_large_kmalloc+0xb3/0x160 mm/slub.c:6352
> Code: 25 00 00 00 ff 3d 00 00 00 f8 0f 85 a6 00 00 00 c7 43 30 ff ff ff ff 48 89 df 44 89 f6 e8 45 d9 fc ff 5b 41 5e 41 5f 5d c3 90 <0f> 0b 90 48 89 df 48 c7 c6 b7 4c 72 8d e8 cb e8 08 ff eb e4 90 0f
> RSP: 0018:ffffc900028e76f8 EFLAGS: 00010287
> RAX: 00000000f0000000 RBX: ffffea00019a5c00 RCX: ffff888067550001
> RDX: 0000000000000000 RSI: ffff888066970000 RDI: ffffea00019a5c00
> RBP: ffffc900028e7710 R08: ffff888049c40603 R09: 1ffff110093880c0
> R10: dffffc0000000000 R11: ffffed10093880c1 R12: ffff888066970000
> R13: ffffffff870bc0f1 R14: 0000000000000000 R15: dffffc0000000000
> FS:  0000000000000000(0000) GS:ffff8880ef136000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007fba7e4bf008 CR3: 000000005776b000 CR4: 00000000000006f0
> 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?

>  request_module_async+0x5e/0x80 drivers/media/usb/em28xx/em28xx-cards.c:3457
>  process_one_work kernel/workqueue.c:3275 [inline]
>  process_scheduled_works+0xae1/0x1800 kernel/workqueue.c:3358
>  worker_thread+0xa0f/0xf70 kernel/workqueue.c:3439
>  kthread+0x37d/0x470 kernel/kthread.c:467
>  ret_from_fork+0x507/0xb90 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:245
>  </TASK>
> 
> <<<<<<<<<<<<<<< tail report >>>>>>>>>>>>>>>
> 
> Modules linked in:
> CPU: 1 UID: 0 PID: 12317 Comm: kworker/1:4 Tainted: G             L      7.0.0-rc1 #1 PREEMPT(full) 
> Tainted: [L]=SOFTLOCKUP
> Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> Workqueue: events request_module_async
> RIP: 0010:free_large_kmalloc+0xb3/0x160
> Code: 25 00 00 00 ff 3d 00 00 00 f8 0f 85 a6 00 00 00 c7 43 30 ff ff ff ff 48 89 df 44 89 f6 e8 45 d9 fc ff 5b 41 5e 41 5f 5d c3 90 <0f> 0b 90 48 89 df 48 c7 c6 b7 4c 72 8d e8 cb e8 08 ff eb e4 90 0f
> RSP: 0018:ffffc900028e76f8 EFLAGS: 00010287
> RAX: 00000000f0000000 RBX: ffffea00019a5c00 RCX: ffff888067550001
> RDX: 0000000000000000 RSI: ffff888066970000 RDI: ffffea00019a5c00
> RBP: ffffc900028e7710 R08: ffff888049c40603 R09: 1ffff110093880c0
> R10: dffffc0000000000 R11: ffffed10093880c1 R12: ffff888066970000
> R13: ffffffff870bc0f1 R14: 0000000000000000 R15: dffffc0000000000
> FS:  0000000000000000(0000) GS:ffff8880ef136000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007fba7e4bf008 CR3: 000000005776b000 CR4: 00000000000006f0
> Call Trace:
>  <TASK>
>  kfree+0xae/0x630
>  usb_free_urb+0xd1/0x120
>  em28xx_uninit_usb_xfer+0x165/0x310
>  em28xx_alloc_urbs+0xf2a/0x1130
>  em28xx_dvb_init+0x2b0/0x4a20
>  em28xx_init_extension+0x121/0x1d0
>  request_module_async+0x5e/0x80
>  process_scheduled_works+0xae1/0x1800
>  worker_thread+0xa0f/0xf70
>  kthread+0x37d/0x470
>  ret_from_fork+0x507/0xb90
>  ret_from_fork_asm+0x11/0x20
>  </TASK>
> Kernel panic - not syncing: kernel: panic_on_warn set ...
> CPU: 1 UID: 0 PID: 12317 Comm: kworker/1:4 Tainted: G             L      7.0.0-rc1 #1 PREEMPT(full) 
> Tainted: [L]=SOFTLOCKUP
> Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> Workqueue: events request_module_async
> Call Trace:
>  <TASK>
>  __dump_stack+0x21/0x30
>  dump_stack_lvl+0x2b/0x150
>  dump_stack+0x19/0x20
>  vpanic+0x53e/0xa20
>  panic+0xb9/0xc0
>  __warn+0x320/0x500
>  __report_bug+0x28d/0x500
>  report_bug+0x175/0x220
>  handle_bug+0x9c/0x200
>  exc_invalid_op+0x1f/0x50
>  asm_exc_invalid_op+0x1f/0x30
> RIP: 0010:free_large_kmalloc+0xb3/0x160
> Code: 25 00 00 00 ff 3d 00 00 00 f8 0f 85 a6 00 00 00 c7 43 30 ff ff ff ff 48 89 df 44 89 f6 e8 45 d9 fc ff 5b 41 5e 41 5f 5d c3 90 <0f> 0b 90 48 89 df 48 c7 c6 b7 4c 72 8d e8 cb e8 08 ff eb e4 90 0f
> RSP: 0018:ffffc900028e76f8 EFLAGS: 00010287
> RAX: 00000000f0000000 RBX: ffffea00019a5c00 RCX: ffff888067550001
> RDX: 0000000000000000 RSI: ffff888066970000 RDI: ffffea00019a5c00
> RBP: ffffc900028e7710 R08: ffff888049c40603 R09: 1ffff110093880c0
> R10: dffffc0000000000 R11: ffffed10093880c1 R12: ffff888066970000
> R13: ffffffff870bc0f1 R14: 0000000000000000 R15: dffffc0000000000
>  kfree+0xae/0x630
>  usb_free_urb+0xd1/0x120
>  em28xx_uninit_usb_xfer+0x165/0x310
>  em28xx_alloc_urbs+0xf2a/0x1130
>  em28xx_dvb_init+0x2b0/0x4a20
>  em28xx_init_extension+0x121/0x1d0
>  request_module_async+0x5e/0x80
>  process_scheduled_works+0xae1/0x1800
>  worker_thread+0xa0f/0xf70
>  kthread+0x37d/0x470
>  ret_from_fork+0x507/0xb90
>  ret_from_fork_asm+0x11/0x20
>  </TASK>
> Kernel Offset: disabled
> Rebooting in 86400 seconds..
> 
> <<<<<<<<<<<<<<< tail report >>>>>>>>>>>>>>>

Re: WARNING in usb_free_urb

[Michal Pecio]

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.

> > 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. 

        /* 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

Re: WARNING in usb_free_urb

[Michal Pecio]

On Mon, 29 Jun 2026 09:20:33 +0200, Michal Pecio wrote:
> 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...

Correction: it failed to recognize the double free, but it fixed a
different (real) problem that the buf array itself was already NULL
and couldn't be scanned for buffer pointers to free.

So it turned this NULL dereferenc into a double free.

Re: WARNING in usb_free_urb

[Vlastimil Babka (SUSE)]

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

Re: WARNING in usb_free_urb

[Hans Verkuil]

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
> 
>