n_tty: Fix echo overrun tail computation

Commit 1a16a04d625642f16886bdb368c780e7a9cab92a,
'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.

Cc: <stable@vger.kernel.org> # 3.12.x : 7e7281b tty: incorrect test of echo_buf() result for ECHO_OP_START
Cc: <stable@vger.kernel.org> # 3.12.x
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 a2b70c97391f28d425472be15500bbf55e151276..d8a779d3a611e5e1106fb36c4c462a17552fe144 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;