This is a note to let you know that I've just added the patch titled
tty: add the option to have a tty reject a new ldisc
to the 6.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
tty-add-the-option-to-have-a-tty-reject-a-new-ldisc.patch
and it can be found in the queue-6.1 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 66ba59dd74fc288fc8f19cc6b642cbba27afd0ba
Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date: Tue Apr 23 09:33:39 2024 -0700
tty: add the option to have a tty reject a new ldisc
[ Upstream commit 6bd23e0c2bb6c65d4f5754d1456bc9a4427fc59b ]
... and use it to limit the virtual terminals to just N_TTY. They are
kind of special, and in particular, the "con_write()" routine violates
the "writes cannot sleep" rule that some ldiscs rely on.
This avoids the
BUG: sleeping function called from invalid context at kernel/printk/printk.c:2659
when N_GSM has been attached to a virtual console, and gsmld_write()
calls con_write() while holding a spinlock, and con_write() then tries
to get the console lock.
Tested-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Jiri Slaby <jirislaby@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Daniel Starke <daniel.starke@xxxxxxxxxxx>
Reported-by: syzbot <syzbot+dbac96d8e73b61aa559c@xxxxxxxxxxxxxxxxxxxxxxxxx>
Closes: https://syzkaller.appspot.com/bug?extid=dbac96d8e73b61aa559c
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20240423163339.59780-1-torvalds@xxxxxxxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 776d8a62f77cc..7ca7731fa78ae 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -546,6 +546,12 @@ int tty_set_ldisc(struct tty_struct *tty, int disc)
goto out;
}
+ if (tty->ops->ldisc_ok) {
+ retval = tty->ops->ldisc_ok(tty, disc);
+ if (retval)
+ goto out;
+ }
+
old_ldisc = tty->ldisc;
/* Shutdown the old discipline. */
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 48a9ed7c93c97..e2f9348725ff1 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -3440,6 +3440,15 @@ static void con_cleanup(struct tty_struct *tty)
tty_port_put(&vc->port);
}
+/*
+ * We can't deal with anything but the N_TTY ldisc,
+ * because we can sleep in our write() routine.
+ */
+static int con_ldisc_ok(struct tty_struct *tty, int ldisc)
+{
+ return ldisc == N_TTY ? 0 : -EINVAL;
+}
+
static int default_color = 7; /* white */
static int default_italic_color = 2; // green (ASCII)
static int default_underline_color = 3; // cyan (ASCII)
@@ -3566,6 +3575,7 @@ static const struct tty_operations con_ops = {
.resize = vt_resize,
.shutdown = con_shutdown,
.cleanup = con_cleanup,
+ .ldisc_ok = con_ldisc_ok,
};
static struct cdev vc0_cdev;
diff --git a/include/linux/tty_driver.h b/include/linux/tty_driver.h
index e00034118c7bc..1df868130adce 100644
--- a/include/linux/tty_driver.h
+++ b/include/linux/tty_driver.h
@@ -155,6 +155,13 @@ struct serial_struct;
*
* Optional. Called under the @tty->termios_rwsem. May sleep.
*
+ * @ldisc_ok: ``int ()(struct tty_struct *tty, int ldisc)``
+ *
+ * This routine allows the @tty driver to decide if it can deal
+ * with a particular @ldisc.
+ *
+ * Optional. Called under the @tty->ldisc_sem and @tty->termios_rwsem.
+ *
* @set_ldisc: ``void ()(struct tty_struct *tty)``
*
* This routine allows the @tty driver to be notified when the device's
@@ -374,6 +381,7 @@ struct tty_operations {
void (*hangup)(struct tty_struct *tty);
int (*break_ctl)(struct tty_struct *tty, int state);
void (*flush_buffer)(struct tty_struct *tty);
+ int (*ldisc_ok)(struct tty_struct *tty, int ldisc);
void (*set_ldisc)(struct tty_struct *tty);
void (*wait_until_sent)(struct tty_struct *tty, int timeout);
void (*send_xchar)(struct tty_struct *tty, char ch);
