tty: Fix oops when scanning the polling list for kgdb

Costantino Leandro found a bug in tty_find_polling_driver and provided a
patch that fixed the crash but not the underlying bug. This fixes the
underlying bug where the list walk corrupts the values it is using on a
match but then reuses them if the open fails.

Signed-off-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 6 deletions

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 66b99a2049e3..6c817398232e 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -295,7 +295,7 @@ struct tty_driver *tty_find_polling_driver(char *name, int *line)
         struct tty_driver *p, *res = NULL;
         int tty_line = 0;
         int len;
-        char *str;
+        char *str, *stp;
 
         for (str = name; *str; str++)
                 if ((*str >= '0' && *str <= '9') || *str == ',')
@@ -311,13 +311,14 @@ struct tty_driver *tty_find_polling_driver(char *name, int *line)
         list_for_each_entry(p, &tty_drivers, tty_drivers) {
                 if (strncmp(name, p->name, len) != 0)
                         continue;
-                if (*str == ',')
-                        str++;
-                if (*str == '\0')
-                        str = NULL;
+                stp = str;
+                if (*stp == ',')
+                        stp++;
+                if (*stp == '\0')
+                        stp = NULL;
 
                 if (tty_line >= 0 && tty_line <= p->num && p->ops &&
-                    p->ops->poll_init && !p->ops->poll_init(p, tty_line, str)) {
+                    p->ops->poll_init && !p->ops->poll_init(p, tty_line, stp)) {
                         res = tty_driver_kref_get(p);
                         *line = tty_line;
                         break;