n_tty: Fix echo overrun tail computation

commit 6f2225363c205e186c1465c2c7c84f17c1635504 upstream.

Commit cbfd0340ae1993378fd47179db949e050e16e697,
'n_tty: Process echoes in blocks', introduced an error when
consuming the echo buffer tail to prevent buffer overrun, where
the incorrect operation code byte is checked to determine how
far to advance the tail to the next echo byte.

Check the correct byte for the echo operation code byte.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 834fb60..f3ef372 100644 (file)
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -768,7 +768,7 @@ static size_t __process_echoes(struct tty_struct *tty)
         * data at the tail to prevent a subsequent overrun */
        while (ldata->echo_commit - tail >= ECHO_DISCARD_WATERMARK) {
                if (echo_buf(ldata, tail) == ECHO_OP_START) {
-                       if (echo_buf(ldata, tail) == ECHO_OP_ERASE_TAB)
+                       if (echo_buf(ldata, tail + 1) == ECHO_OP_ERASE_TAB)
                                tail += 3;
                        else
                                tail += 2;