[RFC] cdc_ncm: fix divide error when USB packet size is 0

[=3D?UTF-8?q?Bj=3DC3=3DB8rn=3D20Mork?=3D]

syzbot reported:

cdc_ncm 1-1:1.0: dwNtbInMaxSize=3D0 is too small. Using 2048
cdc_ncm 1-1:1.0: setting rx_max =3D 2048
cdc_ncm 1-1:1.0: setting tx_max =3D 16384
divide error: 0000 [#1] SMP KASAN
CPU: 0 PID: 12 Comm: kworker/0:1 Not tainted 5.3.0-rc7+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine,
BIOS Google 01/01/2011
Workqueue: usb_hub_wq hub_event
RIP: 0010:cdc_ncm_update_rxtx_max+0xc6e/0xef0 drivers/net/usb/cdc_ncm.c:419
Code: e0 07 38 c2 0f 9e c1 84 d2 0f 95 c0 84 c1 0f 85 35 02 00 00 0f
b7 5b 04 81 e3 ff 07 00 00 e8 29 47 f7 fd 31 d2 44 89 e0 31 ff <f7> f3
41 89 d5 89 d6 e8 86 48 f7 fd 45 85 ed 0f 85 17 f7 ff ff e8
RSP: 0018:ffff8881da20f030 EFLAGS: 00010246
RAX: 0000000000004000 RBX: 0000000000000000 RCX: 0000000000000001
RDX: 0000000000000000 RSI: ffffffff83469377 RDI: 0000000000000000
RBP: ffff8881c7ba0000 R08: ffff8881da1f9800 R09: ffffed103b645d58
R10: ffffed103b645d57 R11: ffff8881db22eabf R12: 0000000000004000
R13: 0000000000000000 R14: ffff8881d35f3dc0 R15: ffff8881d2a70f00
FS:  0000000000000000(0000) GS:ffff8881db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f1059879000 CR3: 00000001d49db000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 cdc_ncm_setup drivers/net/usb/cdc_ncm.c:667 [inline]
 cdc_ncm_bind_common+0x1005/0x2570 drivers/net/usb/cdc_ncm.c:924
 cdc_ncm_bind+0x7c/0x1c0 drivers/net/usb/cdc_ncm.c:1038
 usbnet_probe+0xb43/0x23cf drivers/net/usb/usbnet.c:1722
 usb_probe_interface+0x305/0x7a0 drivers/usb/core/driver.c:361
 really_probe+0x281/0x6d0 drivers/base/dd.c:548
 driver_probe_device+0x101/0x1b0 drivers/base/dd.c:721
 __device_attach_driver+0x1c2/0x220 drivers/base/dd.c:828
 bus_for_each_drv+0x162/0x1e0 drivers/base/bus.c:454
 __device_attach+0x217/0x360 drivers/base/dd.c:894
 bus_probe_device+0x1e4/0x290 drivers/base/bus.c:514
 device_add+0xae6/0x16f0 drivers/base/core.c:2165
 usb_set_configuration+0xdf6/0x1670 drivers/usb/core/message.c:2023
 generic_probe+0x9d/0xd5 drivers/usb/core/generic.c:210
 usb_probe_device+0x99/0x100 drivers/usb/core/driver.c:266
 really_probe+0x281/0x6d0 drivers/base/dd.c:548
 driver_probe_device+0x101/0x1b0 drivers/base/dd.c:721
 __device_attach_driver+0x1c2/0x220 drivers/base/dd.c:828
 bus_for_each_drv+0x162/0x1e0 drivers/base/bus.c:454
 __device_attach+0x217/0x360 drivers/base/dd.c:894
 bus_probe_device+0x1e4/0x290 drivers/base/bus.c:514
 device_add+0xae6/0x16f0 drivers/base/core.c:2165
 usb_new_device.cold+0x6a4/0xe79 drivers/usb/core/hub.c:2536
 hub_port_connect drivers/usb/core/hub.c:5098 [inline]
 hub_port_connect_change drivers/usb/core/hub.c:5213 [inline]
 port_event drivers/usb/core/hub.c:5359 [inline]
 hub_event+0x1b5c/0x3640 drivers/usb/core/hub.c:5441
 process_one_work+0x92b/0x1530 kernel/workqueue.c:2269
 worker_thread+0x96/0xe20 kernel/workqueue.c:2415
 kthread+0x318/0x420 kernel/kthread.c:255
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
Modules linked in:
---[ end trace ab75cc10e099d8e9 ]---

Reported-by: syzbot+ce366e2b8296e25d84f5@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Bj=C3=B8rn Mork <bjorn@xxxxxxx>
---
 drivers/net/usb/cdc_ncm.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
index 50c05d0f44cb..029fe9082a8f 100644
--- a/drivers/net/usb/cdc_ncm.c
+++ b/drivers/net/usb/cdc_ncm.c
@@ -377,6 +377,7 @@ static void cdc_ncm_update_rxtx_max(struct usbnet *dev,=
 u32 new_rx, u32 new_tx)
 {
 	struct cdc_ncm_ctx *ctx =3D (struct cdc_ncm_ctx *)dev->data[0];
 	u8 iface_no =3D ctx->control->cur_altsetting->desc.bInterfaceNumber;
+	u16 maxp =3D usb_maxpacket(dev->udev, dev->out, 1);
 	u32 val;
=20
 	val =3D cdc_ncm_check_rx_max(dev, new_rx);
@@ -411,12 +412,8 @@ static void cdc_ncm_update_rxtx_max(struct usbnet *dev=
, u32 new_rx, u32 new_tx)
 	/* Adding a pad byte here if necessary simplifies the handling
 	 * in cdc_ncm_fill_tx_frame, making tx_max always represent
 	 * the real skb max size.
-	 *
-	 * We cannot use dev->maxpacket here because this is called from
-	 * .bind which is called before usbnet sets up dev->maxpacket
 	 */
-	if (val !=3D le32_to_cpu(ctx->ncm_parm.dwNtbOutMaxSize) &&
-	    val % usb_maxpacket(dev->udev, dev->out, 1) =3D=3D 0)
+	if (maxp && val !=3D le32_to_cpu(ctx->ncm_parm.dwNtbOutMaxSize) && val % =
maxp =3D=3D 0)
 		val++;
=20
 	/* we might need to flush any pending tx buffers if running */
--=20
2.20.1


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