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From:   Xiaomeng Tong <xiam0nd.tong@gmail.com>
To:     hca@linux.ibm.com
Cc:     gor@linux.ibm.com, agordeev@linux.ibm.com,
        borntraeger@linux.ibm.com, svens@linux.ibm.com,
        jirislaby@kernel.org, gregkh@linuxfoundation.org,
        jcmvbkbc@gmail.com, dsterba@suse.com, elder@linaro.org,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        Xiaomeng Tong <xiam0nd.tong@gmail.com>, stable@vger.kernel.org
Subject: [PATCH] char: tty3270: fix a missing check on list iterator
Date:   Sun, 27 Mar 2022 14:49:31 +0800
Message-Id: <20220327064931.7775-1-xiam0nd.tong@gmail.com>
Precedence: bulk

The bug is here:
	if (s->len != flen) {

The list iterator 's' will point to a bogus position containing
HEAD if the list is empty or no element is found. This case must
be checked before any use of the iterator, otherwise it may bpass
the 'if (s->len != flen) {' in theory iif s->len's value is flen.

To fix this bug, use a new variable 'iter' as the list iterator,
while use the origin variable 's' as a dedicated pointer to
point to the found element.

Cc: stable@vger.kernel.org
Fixes: ^1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
---
 drivers/s390/char/tty3270.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/s390/char/tty3270.c b/drivers/s390/char/tty3270.c
index 5c83f71c1d0e..030e9a098d11 100644
--- a/drivers/s390/char/tty3270.c
+++ b/drivers/s390/char/tty3270.c
@@ -1111,7 +1111,7 @@ tty3270_convert_line(struct tty3270 *tp, int line_nr)
 {
 	struct tty3270_line *line;
 	struct tty3270_cell *cell;
-	struct string *s, *n;
+	struct string *s = NULL, *n, *iter;
 	unsigned char highlight;
 	unsigned char f_color;
 	char *cp;
@@ -1142,13 +1142,15 @@ tty3270_convert_line(struct tty3270 *tp, int line_nr)
 
 	/* Find the line in the list. */
 	i = tp->view.rows - 2 - line_nr;
-	list_for_each_entry_reverse(s, &tp->lines, list)
-		if (--i <= 0)
+	list_for_each_entry_reverse(iter, &tp->lines, list)
+		if (--i <= 0) {
+			s = iter;
 			break;
+		}
 	/*
 	 * Check if the line needs to get reallocated.
 	 */
-	if (s->len != flen) {
+	if (!s || s->len != flen) {
 		/* Reallocate string. */
 		n = tty3270_alloc_string(tp, flen);
 		list_add(&n->list, &s->list);
-- 
2.17.1