[PATCH AUTOSEL 6.1 11/12] tty: add the option to have a tty reject a new ldisc

Sasha Levin
2024-06-05T11:54:05Z

From: Linus Torvalds <torvalds@linux-foundation.org>

[ 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@i-love.sakura.ne.jp>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Daniel Starke <daniel.starke@siemens.com>
Reported-by: syzbot <syzbot+dbac96d8e73b61aa559c@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=dbac96d8e73b61aa559c
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240423163339.59780-1-torvalds@linux-foundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/tty/tty_ldisc.c | 6 ++++++
drivers/tty/vt/vt.c | 10 ++++++++++
include/linux/tty_driver.h | 8 ++++++++
3 files changed, 24 insertions(+)

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);
--
2.43.0