KMSAN: uninit-value in asix_mdio_read

Hi,

Following is my analysis of syzbot report:

https://syzkaller.appspot.com/bug?id=66a3f426f314ef275628182228ad31815fac58ed

The call trace is as follows:

 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x191/0x1f0 lib/dump_stack.c:113
 kmsan_report+0x13a/0x2b0 mm/kmsan/kmsan_report.c:108
 __msan_warning+0x73/0xe0 mm/kmsan/kmsan_instr.c:250
 asix_mdio_read+0x3e9/0x8f0 drivers/net/usb/asix_common.c:461
 asix_mdio_bus_read+0xbc/0xe0 drivers/net/usb/ax88172a.c:31
 __mdiobus_read+0x106/0x3d0 drivers/net/phy/mdio_bus.c:563
 mdiobus_read+0xbd/0x110 drivers/net/phy/mdio_bus.c:640
 get_phy_id drivers/net/phy/phy_device.c:785 [inline]
 get_phy_device+0x331/0x8a0 drivers/net/phy/phy_device.c:819
 mdiobus_scan+0x91/0x760 drivers/net/phy/mdio_bus.c:527
 __mdiobus_register+0x86d/0xca0 drivers/net/phy/mdio_bus.c:426
 ax88172a_init_mdio drivers/net/usb/ax88172a.c:105 [inline]
 ax88172a_bind+0xcc5/0xf80 drivers/net/usb/ax88172a.c:243
 usbnet_probe+0x10ae/0x3960 drivers/net/usb/usbnet.c:1722
 usb_probe_interface+0xd19/0x1310 drivers/usb/core/driver.c:361
 really_probe+0x1373/0x1dc0 drivers/base/dd.c:552
 driver_probe_device+0x1ba/0x510 drivers/base/dd.c:709
 __device_attach_driver+0x5b8/0x790 drivers/base/dd.c:816
 bus_for_each_drv+0x28e/0x3b0 drivers/base/bus.c:454
 __device_attach+0x489/0x750 drivers/base/dd.c:882
 device_initial_probe+0x4a/0x60 drivers/base/dd.c:929
 bus_probe_device+0x131/0x390 drivers/base/bus.c:514
 device_add+0x25b5/0x2df0 drivers/base/core.c:2165
 usb_set_configuration+0x309f/0x3710 drivers/usb/core/message.c:2027
 generic_probe+0xe7/0x280 drivers/usb/core/generic.c:210
 usb_probe_device+0x146/0x200 drivers/usb/core/driver.c:266
 really_probe+0x1373/0x1dc0 drivers/base/dd.c:552
 driver_probe_device+0x1ba/0x510 drivers/base/dd.c:709
 __device_attach_driver+0x5b8/0x790 drivers/base/dd.c:816
 bus_for_each_drv+0x28e/0x3b0 drivers/base/bus.c:454
 __device_attach+0x489/0x750 drivers/base/dd.c:882
 device_initial_probe+0x4a/0x60 drivers/base/dd.c:929
 bus_probe_device+0x131/0x390 drivers/base/bus.c:514
 device_add+0x25b5/0x2df0 drivers/base/core.c:2165
 usb_new_device+0x23e5/0x2fb0 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+0x581d/0x72f0 drivers/usb/core/hub.c:5441
 process_one_work+0x1572/0x1ef0 kernel/workqueue.c:2269
 worker_thread+0x111b/0x2460 kernel/workqueue.c:2415
 kthread+0x4b5/0x4f0 kernel/kthread.c:256
 ret_from_fork+0x35/0x40 arch/x86/entry/entry_64.S:355

drivers/net/usb/asix_common.c:461 points to this:

	u8 smsr;
	// ...
	do {
		ret = asix_set_sw_mii(dev, 0);
		if (ret == -ENODEV || ret == -ETIMEDOUT)
			break;
		usleep_range(1000, 1100);
		ret = asix_read_cmd(dev, AX_CMD_STATMNGSTS_REG,
				    0, 0, 1, &smsr, 0);

-> } while (!(smsr & AX_HOST_EN) && (i++ < 30) && (ret != -ENODEV));

i.e. the variable "smsr", as pointed out by the tail of the syzbot report. Now, a read command is initiated at the end of the while loop. asix_read_cmd calls usbnet_read_cmd, which fills up the buffer only on one code path, as follows:

	err = usb_control_msg(dev->udev, usb_rcvctrlpipe(dev->udev, 0),
			      cmd, reqtype, value, index, buf, size,
			      USB_CTRL_GET_TIMEOUT);
	if (err > 0 && err <= size) {
        if (data)
            memcpy(data, buf, err);

So most probably a negative error code was generated by usb_control_msg, as evinced by the logs in the head of the syzbot report, which led to the buffer being left uninitialized. The while condition was then inevitably executed, and thus generated the error.

The most apparent fix for this is to initialize smsr to 1.

- u8 smsr;
+ u8 smsr = 1;

Thus, even if smsr is untouched by the usbnet function calls, the first condition of the while loop won't evaluate to true anyway.

Similar changes should be made to other ASIX command functions in drivers/net/usb/asix_common.c.

Cheers,
Jaskaran.