Re: usb/net/qmi_wwan: divide error in qmi_wwan_probe/usbnet_probe

["Bjørn Mork" <bjorn@mork.no>]

Andrey Konovalov <andreyknvl@google.com> writes:

> Hi!
>
> I've got the following report while fuzzing the kernel with syzkaller.

Thanks.  It would have helped a lot of you said *what* you were fuzzing,
though.... But based on where the bug is, I assume it is USB
descriptors?


> On commit 39dae59d66acd86d1de24294bd2f343fd5e7a625 (4.14-rc8).
>
> qmi_wwan 1-1:0.4: cdc-wdm0: USB WDM device
> divide error: 0000 [#1] PREEMPT SMP KASAN
> Modules linked in:
> CPU: 0 PID: 24 Comm: kworker/0:1 Not tainted 4.14.0-rc8-44453-g1fdc1a82c34f #56
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> Workqueue: usb_hub_wq hub_event
> task: ffff88006bef5c00 task.stack: ffff88006bf60000
> RIP: 0010:usbnet_update_max_qlen+0x24d/0x390 drivers/net/usb/usbnet.c:355
> RSP: 0018:ffff88006bf67508 EFLAGS: 00010246
> RAX: 00000000000163c8 RBX: ffff8800621fce40 RCX: ffff8800621fcf34
> RDX: 0000000000000000 RSI: ffffffff837ecb7a RDI: ffff8800621fcf34
> RBP: ffff88006bf67520 R08: ffff88006bef5c00 R09: ffffed000c43f881
> R10: ffffed000c43f880 R11: ffff8800621fc406 R12: 0000000000000003
> R13: ffffffff85c71de0 R14: 0000000000000000 R15: 0000000000000000
> FS:  0000000000000000(0000) GS:ffff88006ca00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007ffe9c0d6dac CR3: 00000000614f4000 CR4: 00000000000006f0
> Call Trace:
>  usbnet_probe+0x18b5/0x2790 drivers/net/usb/usbnet.c:1783
>  qmi_wwan_probe+0x133/0x220 drivers/net/usb/qmi_wwan.c:1338

So, looking over this again and again, the only obviously risky division
I can see is the usbnet_update_max_qlen() call where we divide on both
dev->rx_urb_size and dev->hard_mtu.

I don't think dev->rx_urb_size can be 0 unless dev->hard_mtu is 0.  But
the latter is possible, and this is a bug. In qmi_wwan_bind(), which is
called by usbnet_probe() prior to the usbnet_update_max_qlen() call, we
do

	if (cdc_ether) {
		dev->hard_mtu = le16_to_cpu(cdc_ether->wMaxSegmentSize);
		usbnet_get_ethernet_addr(dev, cdc_ether->iMACAddress);
	}

which will be fatal if cdc_ether->wMaxSegmentSize is 0.  I assume that
is what your fuzzer did?  Fix coming up RSN.  Thanks a lot for pointing
this out.


Bjørn