tty: Prevent writing chars during tcsetattr TCSADRAIN/FLUSH

If userspace races tcsetattr() with a write, the drained condition
might not be guaranteed by the kernel. There is a race window after
checking Tx is empty before tty_set_termios() takes termios_rwsem for
write. During that race window, more characters can be queued by a
racing writer.

Any ongoing transmission might produce garbage during HW's
->set_termios() call. The intent of TCSADRAIN/FLUSH seems to be
preventing such a character corruption. If those flags are set, take
tty's write lock to stop any writer before performing the lower layer
Tx empty check and wait for the pending characters to be sent (if any).

The initial wait for all-writers-done must be placed outside of tty's
write lock to avoid deadlock which makes it impossible to use
tty_wait_until_sent(). The write lock is retried if a racing write is
detected.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20230317113318.31327-2-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 094fb49a2d0d6827c86d2e0840873e6db0c491d2)
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 37c46c65a65f4df8786ca6f900c343ba35d40243..bb14bd4f6e55d97a8131a9e345cb6398c08ce354 100644 (file)
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -876,13 +876,13 @@ static ssize_t tty_read(struct file *file, char __user *buf, size_t count,
        return i;
 }
 
-static void tty_write_unlock(struct tty_struct *tty)
+void tty_write_unlock(struct tty_struct *tty)
 {
        mutex_unlock(&tty->atomic_write_lock);
        wake_up_interruptible_poll(&tty->write_wait, EPOLLOUT);
 }
 
-static int tty_write_lock(struct tty_struct *tty, int ndelay)
+int tty_write_lock(struct tty_struct *tty, int ndelay)
 {
        if (!mutex_trylock(&tty->atomic_write_lock)) {
                if (ndelay)

diff --git a/drivers/tty/tty_ioctl.c b/drivers/tty/tty_ioctl.c
index 9245fffdbceb170c1e686c093194f48d7f50ac49..5d833f0b412d5407b86ad920ebd5a0adda58d00a 100644 (file)
--- a/drivers/tty/tty_ioctl.c
+++ b/drivers/tty/tty_ioctl.c
@@ -397,21 +397,42 @@ static int set_termios(struct tty_struct *tty, void __user *arg, int opt)
        tmp_termios.c_ispeed = tty_termios_input_baud_rate(&tmp_termios);
        tmp_termios.c_ospeed = tty_termios_baud_rate(&tmp_termios);
 
-       ld = tty_ldisc_ref(tty);
+       if (opt & (TERMIOS_FLUSH|TERMIOS_WAIT)) {
+retry_write_wait:
+               retval = wait_event_interruptible(tty->write_wait, !tty_chars_in_buffer(tty));
+               if (retval < 0)
+                       return retval;
 
-       if (ld != NULL) {
-               if ((opt & TERMIOS_FLUSH) && ld->ops->flush_buffer)
-                       ld->ops->flush_buffer(tty);
-               tty_ldisc_deref(ld);
-       }
+               if (tty_write_lock(tty, 0) < 0)
+                       goto retry_write_wait;
 
-       if (opt & TERMIOS_WAIT) {
-               tty_wait_until_sent(tty, 0);
-               if (signal_pending(current))
-                       return -ERESTARTSYS;
-       }
+               /* Racing writer? */
+               if (tty_chars_in_buffer(tty)) {
+                       tty_write_unlock(tty);
+                       goto retry_write_wait;
+               }
+
+               ld = tty_ldisc_ref(tty);
+               if (ld != NULL) {
+                       if ((opt & TERMIOS_FLUSH) && ld->ops->flush_buffer)
+                               ld->ops->flush_buffer(tty);
+                       tty_ldisc_deref(ld);
+               }
+
+               if ((opt & TERMIOS_WAIT) && tty->ops->wait_until_sent) {
+                       tty->ops->wait_until_sent(tty, 0);
+                       if (signal_pending(current)) {
+                               tty_write_unlock(tty);
+                               return -ERESTARTSYS;
+                       }
+               }
+
+               tty_set_termios(tty, &tmp_termios);
 
-       tty_set_termios(tty, &tmp_termios);
+               tty_write_unlock(tty);
+       } else {
+               tty_set_termios(tty, &tmp_termios);
+       }
 
        /* FIXME: Arguably if tmp_termios == tty->termios AND the
           actual requested termios was not tmp_termios then we may

diff --git a/include/linux/tty.h b/include/linux/tty.h
index eb33d948788cc3ceed5a08ac1963fd05a5eff3f5..8be25caa97f67f2b1d4032f19ca167e99e26d7c5 100644 (file)
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -480,6 +480,8 @@ extern void __stop_tty(struct tty_struct *tty);
 extern void stop_tty(struct tty_struct *tty);
 extern void __start_tty(struct tty_struct *tty);
 extern void start_tty(struct tty_struct *tty);
+void tty_write_unlock(struct tty_struct *tty);
+int tty_write_lock(struct tty_struct *tty, int ndelay);
 extern int tty_register_driver(struct tty_driver *driver);
 extern int tty_unregister_driver(struct tty_driver *driver);
 extern struct device *tty_register_device(struct tty_driver *driver,