serdev: ttyport: add missing receive_buf sanity checks

The receive_buf tty-port callback should return the number of bytes
accepted and must specifically never return a negative errno (or a value
larger than the buffer size) to the tty layer.

A serdev driver not providing a receive_buf callback would currently
cause the flush_to_ldisc() worker to spin in a tight loop when the tty
buffer pointers are incremented with -EINVAL (-22) after data has been
received.

A serdev driver occasionally returning a negative errno (or a too large
byte count) could cause information leaks or crashes when accessing
memory outside the tty buffers in consecutive callbacks.

Fixes: cd6484e1830b ("serdev: Introduce new bus for serial attached devices")
Cc: stable <stable@vger.kernel.org>	# 4.11
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 12 insertions, 1 deletions

diff --git a/drivers/tty/serdev/serdev-ttyport.c b/drivers/tty/serdev/serdev-ttyport.c
index ce7ad0acee7a..09fbdd52a561 100644
--- a/drivers/tty/serdev/serdev-ttyport.c
+++ b/drivers/tty/serdev/serdev-ttyport.c
@@ -27,11 +27,22 @@ static int ttyport_receive_buf(struct tty_port *port, const unsigned char *cp,
 {
         struct serdev_controller *ctrl = port->client_data;
         struct serport *serport = serdev_controller_get_drvdata(ctrl);
+        int ret;
 
         if (!test_bit(SERPORT_ACTIVE, &serport->flags))
                 return 0;
 
-        return serdev_controller_receive_buf(ctrl, cp, count);
+        ret = serdev_controller_receive_buf(ctrl, cp, count);
+
+        dev_WARN_ONCE(&ctrl->dev, ret < 0 || ret > count,
+                                "receive_buf returns %d (count = %zu)\n",
+                                ret, count);
+        if (ret < 0)
+                return 0;
+        else if (ret > count)
+                return count;
+
+        return ret;
 }
 
 static void ttyport_write_wakeup(struct tty_port *port)