FAILED: patch "[PATCH] tty: Prevent writing chars during tcsetattr TCSADRAIN/FLUSH" failed to apply to 4.19-stable tree

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-4.19.y
git checkout FETCH_HEAD
git cherry-pick -x 094fb49a2d0d6827c86d2e0840873e6db0c491d2
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2023050618-vowel-angular-3166@gregkh' --subject-prefix 'PATCH 4.19.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 094fb49a2d0d6827c86d2e0840873e6db0c491d2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ilpo=20J=C3=A4rvinen?= <[email protected]>
Date: Fri, 17 Mar 2023 13:33:17 +0200
Subject: [PATCH] tty: Prevent writing chars during tcsetattr TCSADRAIN/FLUSH
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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.

index f45cd683c02e..1e0d80e98d26 100644
@@ -62,6 +62,8 @@ int __tty_check_change(struct tty_struct *tty, int sig);
 int tty_check_change(struct tty_struct *tty);
 void __stop_tty(struct tty_struct *tty);
 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);
 void tty_vhangup_session(struct tty_struct *tty);
 void tty_open_proc_set_tty(struct file *filp, struct tty_struct *tty);
 int tty_signal_session_leader(struct tty_struct *tty, int exit_session);

index 766750e355ac..cfb3da0dee47 100644
@@ -933,13 +933,13 @@ static ssize_t tty_read(struct kiocb *iocb, struct iov_iter *to)
 	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)