Subject: [PATCH] net: mcs7830: handle usb read errors properly

From: Pavel Skripkin <[email-masked]>
Date: Fri, 7 Jan 2022 01:57:16 +0300

Syzbot reported uninit value in mcs7830_bind(). The problem was in
missing validation check for bytes read via usbnet_read_cmd().

usbnet_read_cmd() internally calls usb_control_msg(), that returns
number of bytes read. Code should validate that requested number of bytes
was actually read.

So, this patch adds missing size validation check inside
mcs7830_get_reg() to prevent uninit value bugs

CC: Arnd Bergmann <[email-masked]>
Reported-and-tested-by: [email-masked]
Fixes: 2a36d7083438 ("USB: driver for mcs7830 (aka DeLOCK) USB ethernet adapter")
Signed-off-by: Pavel Skripkin <[email-masked]>
---

@Arnd, I am not sure about mcs7830_get_rev() function. 

Is get_reg(22, 2) == 1 valid read? If so, I think, we should call
usbnet_read_cmd() directly here, since other callers care only about
negative error values.  

Thanks

---
 drivers/net/usb/mcs7830.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/net/usb/mcs7830.c b/drivers/net/usb/mcs7830.c
index 326cc4e749d8..fdda0616704e 100644
--- a/drivers/net/usb/mcs7830.c
+++ b/drivers/net/usb/mcs7830.c
@@ -108,8 +108,16 @@ static const char driver_name[] = "MOSCHIP usb-ethernet driver";
 
 static int mcs7830_get_reg(struct usbnet *dev, u16 index, u16 size, void *data)
 {
-	return usbnet_read_cmd(dev, MCS7830_RD_BREQ, MCS7830_RD_BMREQ,
-				0x0000, index, data, size);
+	int ret;
+
+	ret = usbnet_read_cmd(dev, MCS7830_RD_BREQ, MCS7830_RD_BMREQ,
+			      0x0000, index, data, size);
+	if (ret < 0)
+		return ret;
+	else if (ret < size)
+		return -ENODATA;
+
+	return ret;
 }
 
 static int mcs7830_set_reg(struct usbnet *dev, u16 index, u16 size, const void *data)
-- 
2.34.1