[PATCH] um: ubd: Fix crash from option parsing

[Paul Lawrence]

Below patch will cause NULL ptr dereferences if the optional filenames
are not present.

Fixes: ef3ba87cb7c9 (um: ubd: Set device serial attribute from cmdline)
Signed-off-by: Paul Lawrence <paullawrence at google.com>
---
 arch/um/drivers/ubd_kern.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/um/drivers/ubd_kern.c b/arch/um/drivers/ubd_kern.c
index 13b1fe694b90..704989088f28 100644
--- a/arch/um/drivers/ubd_kern.c
+++ b/arch/um/drivers/ubd_kern.c
@@ -371,15 +371,15 @@ static int ubd_setup_common(char *str, int *index_out, char **error_out)
 
 break_loop:
 	file = strsep(&str, ",:");
-	if (*file == '\0')
+	if (file && *file == '\0')
 		file = NULL;
 
 	backing_file = strsep(&str, ",:");
-	if (*backing_file == '\0')
+	if (backing_file && *backing_file == '\0')
 		backing_file = NULL;
 
 	serial = strsep(&str, ",:");
-	if (*serial == '\0')
+	if (serial && *serial == '\0')
 		serial = NULL;
 
 	if (backing_file && ubd_dev->no_cow) {
-- 
2.30.0.284.gd98b1dd5eaa7-goog