auxdisplay: line-display: fix NULL dereference in linedisp_release

linedisp_release() currently retrieves the enclosing struct linedisp via
to_linedisp(). That lookup depends on the attachment list, but the
attachment may already have been removed before put_device() invokes the
release callback. This can happen in linedisp_unregister(), and can also
be reached from some linedisp_register() error paths.

In that case, to_linedisp() returns NULL and linedisp_release()
dereferences it while freeing the display resources.

The struct device released here is the embedded linedisp->dev used by
linedisp_register(), so retrieve the enclosing object directly with
container_of() instead.

Fixes: 66c93809487e ("auxdisplay: linedisp: encapsulate container_of usage within to_linedisp")
Cc: stable@vger.kernel.org
Signed-off-by: Guangshuo Li <lgs201920130244@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/auxdisplay/line-display.c

diff --git a/drivers/auxdisplay/line-display.c b/drivers/auxdisplay/line-display.c
index 81b4aac..fb6d9294 100644
--- a/drivers/auxdisplay/line-display.c
+++ b/drivers/auxdisplay/line-display.c
@@ -365,7 +365,7 @@ static DEFINE_IDA(linedisp_id);
 
 static void linedisp_release(struct device *dev)
 {
-	struct linedisp *linedisp = to_linedisp(dev);
+	struct linedisp *linedisp = container_of(dev, struct linedisp, dev);
 
 	kfree(linedisp->map);
 	kfree(linedisp->message);