[PATCH v2 09/10] HID: steam: Reject short reads

[Vicki Pfau <vi@endrift.com>]

Steam Controller FEATURE reports encode the size of the message in the
message itself. Previously we were trusting that the size reported matched
the size we actually read, leading to a potential issue with short reads.
Instead, we should actually verify the length of the read.

Fixes: c164d6abf384 ("HID: add driver for Valve Steam Controller")
Reported-by: syzbot+75f3f9bff8c510602d36@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=75f3f9bff8c510602d36

Signed-off-by: Vicki Pfau <vi@endrift.com>
---
 drivers/hid/hid-steam.c | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/drivers/hid/hid-steam.c b/drivers/hid/hid-steam.c
index 1f2cf15d4083..49f6733d8935 100644
--- a/drivers/hid/hid-steam.c
+++ b/drivers/hid/hid-steam.c
@@ -357,6 +357,13 @@ static int steam_recv_report(struct steam_device *steam,
 	u8 *buf;
 	int ret;
 
+	/*
+	 * All reports start with a two byte header.
+	 * We must read at least two bytes to get a sensible output.
+	 */
+	if (size < 2)
+		return -EINVAL;
+
 	r = steam->hdev->report_enum[HID_FEATURE_REPORT].report_id_hash[0];
 	if (!r) {
 		hid_err(steam->hdev, "No HID_FEATURE_REPORT submitted -  nothing to read\n");
@@ -380,8 +387,10 @@ static int steam_recv_report(struct steam_device *steam,
 			buf, hid_report_len(r) + 1,
 			HID_FEATURE_REPORT, HID_REQ_GET_REPORT);
 	if (ret > 0) {
-		ret = min(size, ret - 1);
-		memcpy(data, buf + 1, ret);
+		/* Remove the report ID from the return buffer */
+		ret--;
+		size = min(size, ret);
+		memcpy(data, buf + 1, size);
 	}
 	kfree(buf);
 
@@ -389,7 +398,15 @@ static int steam_recv_report(struct steam_device *steam,
 		hid_err(steam->hdev, "%s: error %d\n", __func__, ret);
 	else
 		hid_dbg(steam->hdev, "Received report %*ph\n", ret, data);
-	return ret;
+	if (ret < 0)
+		return ret;
+
+	if (ret < 2 || ret < data[1] + 2) {
+		hid_err(steam->hdev, "%s: expected %u bytes, read %i\n",
+				__func__, data[1] + 2, ret);
+		return -EPROTO;
+	}
+	return size;
 }
 
 static int steam_send_report(struct steam_device *steam,
-- 
2.54.0