From 094fb49a2d0d6827c86d2e0840873e6db0c491d2 Mon Sep 17 00:00:00 2001 From: Ilpo Järvinen Date: Fri, 17 Mar 2023 13:33:17 +0200 Subject: tty: Prevent writing chars during tcsetattr TCSADRAIN/FLUSH MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Link: https://lore.kernel.org/r/20230317113318.31327-2-ilpo.jarvinen@linux.intel.com Signed-off-by: Greg Kroah-Hartman --- drivers/tty/tty_ioctl.c | 45 +++++++++++++++++++++++++++++++++------------ 1 file changed, 33 insertions(+), 12 deletions(-) (limited to 'drivers/tty/tty_ioctl.c') diff --git a/drivers/tty/tty_ioctl.c b/drivers/tty/tty_ioctl.c index 32ff9959b565..2e88b414cf95 100644 --- a/drivers/tty/tty_ioctl.c +++ b/drivers/tty/tty_ioctl.c @@ -501,21 +501,42 @@ 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; + } - tty_set_termios(tty, &tmp_termios); + 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_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 -- cgit v1.2.3