Input: ims-pcu - fix heap-buffer-overflow in ims_pcu_process_data()

The `ims_pcu_process_data()` processes incoming URB data byte by byte.
However, it fails to check if the `read_pos` index exceeds
IMS_PCU_BUF_SIZE.

If a malicious USB device sends a packet larger than IMS_PCU_BUF_SIZE,
`read_pos` will increment indefinitely. Moreover, since `read_pos` is
located immediately after `read_buf`, the attacker can overwrite
`read_pos` itself to arbitrarily control the index.

This manipulated `read_pos` is subsequently used in
`ims_pcu_handle_response()` to copy data into `cmd_buf`, leading to a
heap buffer overflow.

Specifically, an attacker can overwrite the `cmd_done.wait.head` located
at offset 136 relative to `cmd_buf` in the `ims_pcu_handle_response()`.
Consequently, when the driver calls `complete(&pcu->cmd_done)`, it
triggers a control flow hijack by using the manipulated pointer.

Fix this by adding a bounds check for `read_pos` before writing to
`read_buf`. If the packet is too long, discard it, log a warning,
and reset the parser state.

Fixes: 628329d ("Input: add IMS Passenger Control Unit driver")
Co-developed-by: Sanghoon Choi <[email protected]>
Signed-off-by: Sanghoon Choi <[email protected]>
Signed-off-by: Seungjin Bae <[email protected]>
Link: https://patch.msgid.link/[email protected]
[dtor: factor out resetting packet state, reset checksum as well]
Signed-off-by: Dmitry Torokhov <[email protected]>

Author: pip-izony

drivers/input/misc/ims-pcu.c

@@ -438,6 +438,14 @@ static void ims_pcu_handle_response(struct ims_pcu *pcu)
 	}
 }
 
+static void ims_pcu_reset_packet(struct ims_pcu *pcu)
+{
+	pcu->have_stx = true;
+	pcu->have_dle = false;
+	pcu->read_pos = 0;
+	pcu->check_sum = 0;
+}
+
 static void ims_pcu_process_data(struct ims_pcu *pcu, struct urb *urb)
 {
 	int i;
@@ -450,6 +458,14 @@ static void ims_pcu_process_data(struct ims_pcu *pcu, struct urb *urb)
 			continue;
 
 		if (pcu->have_dle) {
+			if (pcu->read_pos >= IMS_PCU_BUF_SIZE) {
+				dev_warn(pcu->dev,
+					 "Packet too long (%d bytes), discarding\n",
+					 pcu->read_pos);
+				ims_pcu_reset_packet(pcu);
+				continue;
+			}
+
 			pcu->have_dle = false;
 			pcu->read_buf[pcu->read_pos++] = data;
 			pcu->check_sum += data;
@@ -462,10 +478,8 @@ static void ims_pcu_process_data(struct ims_pcu *pcu, struct urb *urb)
 				dev_warn(pcu->dev,
 					 "Unexpected STX at byte %d, discarding old data\n",
 					 pcu->read_pos);
+			ims_pcu_reset_packet(pcu);
 			pcu->have_stx = true;
-			pcu->have_dle = false;
-			pcu->read_pos = 0;
-			pcu->check_sum = 0;
 			break;
 
 		case IMS_PCU_PROTOCOL_DLE:
@@ -485,12 +499,18 @@ static void ims_pcu_process_data(struct ims_pcu *pcu, struct urb *urb)
 				ims_pcu_handle_response(pcu);
 			}
 
-			pcu->have_stx = false;
-			pcu->have_dle = false;
-			pcu->read_pos = 0;
+			ims_pcu_reset_packet(pcu);
 			break;
 
 		default:
+			if (pcu->read_pos >= IMS_PCU_BUF_SIZE) {
+				dev_warn(pcu->dev,
+					 "Packet too long (%d bytes), discarding\n",
+					 pcu->read_pos);
+				ims_pcu_reset_packet(pcu);
+				continue;
+			}
+
 			pcu->read_buf[pcu->read_pos++] = data;
 			pcu->check_sum += data;
 			break;