n_tty: signal and flush atomically

When handling signalling char, claim the termios write lock before
signalling waiting readers and writers to prevent further i/o
before flushing the echo and output buffers. This prevents a
userspace signal handler which may output from racing the terminal
flush.

Reference: Bugzilla #99351 ("Output truncated in ssh session after...")
Fixes: commit d2b6f44779d3 ("n_tty: Fix signal handling flushes")
Reported-by: Filipe Brandenburger <filbranden@google.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 13 insertions, 3 deletions

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index c9c27f69e101..ee8bfacf2071 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1108,19 +1108,29 @@ static void eraser(unsigned char c, struct tty_struct *tty)
  *      Locking: ctrl_lock
  */
 
-static void isig(int sig, struct tty_struct *tty)
+static void __isig(int sig, struct tty_struct *tty)
 {
-        struct n_tty_data *ldata = tty->disc_data;
         struct pid *tty_pgrp = tty_get_pgrp(tty);
         if (tty_pgrp) {
                 kill_pgrp(tty_pgrp, sig, 1);
                 put_pid(tty_pgrp);
         }
+}
 
-        if (!L_NOFLSH(tty)) {
+static void isig(int sig, struct tty_struct *tty)
+{
+        struct n_tty_data *ldata = tty->disc_data;
+
+        if (L_NOFLSH(tty)) {
+                /* signal only */
+                __isig(sig, tty);
+
+        } else { /* signal and flush */
                 up_read(&tty->termios_rwsem);
                 down_write(&tty->termios_rwsem);
 
+                __isig(sig, tty);
+
                 /* clear echo buffer */
                 mutex_lock(&ldata->output_lock);
                 ldata->echo_head = ldata->echo_tail = 0;