[PATCH] HID: mcp2221: reject short input reports

[Yousef Alhouseen <alhouseenyousef@gmail.com>]

The MCP2221 raw-event callback reads fixed offsets from the 64-byte input
report, including data[50] and a variable payload beginning at data[4].
Raw-event callbacks run before HID core extends short reports to their
declared size, so a malformed USB device can make these accesses run past
the received buffer.

Reject reports whose size does not match the protocol and complete the
pending command with -EMSGSIZE so its caller does not wait for a timeout.

Fixes: 67a95c21463d ("HID: mcp2221: add usb to i2c-smbus host bridge")
Reported-by: syzbot+1018672fe70298606e5f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=1018672fe70298606e5f
Cc: stable@vger.kernel.org
Signed-off-by: Yousef Alhouseen <alhouseenyousef@gmail.com>
---
 drivers/hid/hid-mcp2221.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/hid/hid-mcp2221.c b/drivers/hid/hid-mcp2221.c
index e4ddd8e9293b..311b51c17b4e 100644
--- a/drivers/hid/hid-mcp2221.c
+++ b/drivers/hid/hid-mcp2221.c
@@ -861,6 +861,12 @@ static int mcp2221_raw_event(struct hid_device *hdev,
 	u8 *buf;
 	struct mcp2221 *mcp = hid_get_drvdata(hdev);
 
+	if (size != sizeof(mcp->txbuf)) {
+		mcp->status = -EMSGSIZE;
+		complete(&mcp->wait_in_report);
+		return 1;
+	}
+
 	switch (data[0]) {
 
 	case MCP2221_I2C_WR_DATA:
-- 
2.54.0