[PATCH] HID: hid-lg4ff: Add check for empty lbuf

[Anastasia Belova]

If an empty buf is received, lbuf is also empty. So lbuf is 
accessed by index -1.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: f31a2de3fe36 ("HID: hid-lg4ff: Allow switching of Logitech gaming wheels between compatibility modes")

Signed-off-by: Anastasia Belova <abelova@xxxxxxxxxxxxx>
---
 drivers/hid/hid-lg4ff.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/hid/hid-lg4ff.c b/drivers/hid/hid-lg4ff.c
index 5e6a0cef2a06..e3fcf1353fb3 100644
--- a/drivers/hid/hid-lg4ff.c
+++ b/drivers/hid/hid-lg4ff.c
@@ -872,6 +872,12 @@ static ssize_t lg4ff_alternate_modes_store(struct device *dev, struct device_att
 		return -ENOMEM;
 
 	i = strlen(lbuf);
+
+	if (i == 0) {
+		kfree(lbuf);
+		return -EINVAL;
+	}
+
 	if (lbuf[i-1] == '\n') {
 		if (i == 1) {
 			kfree(lbuf);
-- 
2.30.2