netdev - Re: KMSAN: uninit-value in smsc75xx

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Date:   Tue, 13 Aug 2019 14:43:40 +0200
From:   Oliver Neukum <oneukum@...e.com>
To:     syzbot <syzbot+6966546b78d050bb0b5d@...kaller.appspotmail.com>,
        davem@...emloft.net, glider@...gle.com,
        syzkaller-bugs@...glegroups.com, steve.glendinning@...well.net,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: KMSAN: uninit-value in smsc75xx_bind

Am Freitag, den 09.08.2019, 01:48 -0700 schrieb syzbot:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    beaab8a3 fix KASAN build
> git tree:       kmsan

[..]
> Call Trace:
>   __dump_stack lib/dump_stack.c:77 [inline]
>   dump_stack+0x191/0x1f0 lib/dump_stack.c:113
>   kmsan_report+0x162/0x2d0 mm/kmsan/kmsan_report.c:109
>   __msan_warning+0x75/0xe0 mm/kmsan/kmsan_instr.c:294
>   smsc75xx_wait_ready drivers/net/usb/smsc75xx.c:976 [inline]
>   smsc75xx_bind+0x541/0x12d0 drivers/net/usb/smsc75xx.c:1483

> 
> Local variable description: ----buf.i93@...c75xx_bind
> Variable was created at:
>   __smsc75xx_read_reg drivers/net/usb/smsc75xx.c:83 [inline]
>   smsc75xx_wait_ready drivers/net/usb/smsc75xx.c:969 [inline]
>   smsc75xx_bind+0x44c/0x12d0 drivers/net/usb/smsc75xx.c:1483
>   usbnet_probe+0x10d3/0x3950 drivers/net/usb/usbnet.c:1722

Hi,

this looks like a false positive to me.
The offending code is likely this:

        if (size) {
                buf = kmalloc(size, GFP_KERNEL);
                if (!buf)
                        goto out;
        }

        err = usb_control_msg(dev->udev, usb_rcvctrlpipe(dev->udev, 0),
                              cmd, reqtype, value, index, buf, size,
                              USB_CTRL_GET_TIMEOUT);

which uses 'buf' uninitialized. But it is used for input.
What is happening here?

	Regards
		Oliver