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tty: add the option to have a tty reject a new ldisc

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... 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>
This commit is contained in:
Linus Torvalds 2024-04-23 09:33:39 -07:00 committed by Greg Kroah-Hartman
parent a47cf07f60
commit 6bd23e0c2b
3 changed files with 24 additions and 0 deletions
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6
drivers/tty/tty_ldisc.c
View File

@ -545,6 +545,12 @@ 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. */

10
drivers/tty/vt/vt.c
View File

@ -3576,6 +3576,15 @@ 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)
@ -3695,6 +3704,7 @@ 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;

8
include/linux/tty_driver.h
View File

@ -154,6 +154,13 @@ 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
@ -372,6 +379,7 @@ 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, u8 ch);