[PATCH] HID: mcp2221: prevent a buffer overflow in mcp_smbus_write()

[Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>]

Smatch Warning:
drivers/hid/hid-mcp2221.c:388 mcp_smbus_write() error: __memcpy()
'&mcp->txbuf[5]' too small (59 vs 255)
drivers/hid/hid-mcp2221.c:388 mcp_smbus_write() error: __memcpy() 'buf'
too small (34 vs 255)

The 'len' variable can take a value between 0-255 as it can come from
data->block[0] and it is user data. So add an bound check to prevent a
buffer overflow in memcpy().

Fixes: 67a95c21463d ("HID: mcp2221: add usb to i2c-smbus host bridge")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
---
I believe I2C_SMBUS_BLOCK_MAX (32) is the appropriate limit to use here
but the &mcp->txbuf[5] array could actually fit 59 bytes which is the
destination in this case. I don't know why the buffer is larger than
expected.

 drivers/hid/hid-mcp2221.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/hid/hid-mcp2221.c b/drivers/hid/hid-mcp2221.c
index 4211b9839209..de52e9f7bb8c 100644
--- a/drivers/hid/hid-mcp2221.c
+++ b/drivers/hid/hid-mcp2221.c
@@ -385,6 +385,9 @@ static int mcp_smbus_write(struct mcp2221 *mcp, u16 addr,
 		data_len = 7;
 		break;
 	default:
+		if (len > I2C_SMBUS_BLOCK_MAX)
+			return -EINVAL;
+
 		memcpy(&mcp->txbuf[5], buf, len);
 		data_len = len + 5;
 	}
-- 
2.31.1