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Commit 8a8dabf2 authored by Alan Cox's avatar Alan Cox Committed by Greg Kroah-Hartman
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tty: handle the case where we cannot restore a line discipline 



Historically the N_TTY driver could never fail but this has become broken over
time. Rather than trying to rewrite half the ldisc layer to fix the breakage
introduce a second level of fallback with an N_NULL ldisc which cannot fail,
and thus restore the guarantees required by the ldisc layer.

We still try and fail to N_TTY first. It's much more useful to find yourself
back in your old ldisc (first attempt) or in N_TTY (second attempt), and while
I'm not aware of any code out there that makes those assumptions it's good to
drive(r) defensively.

Signed-off-by: default avatarAlan Cox <alan@linux.intel.com>
Reported-by: default avatarDmitry Vyukov <dvyukov@google.com>
Tested-by: default avatarDmitry Vyukov <dvyukov@google.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 47f58e32

drivers/tty/Makefile

+2 −1
Original line number Diff line number Diff line 
obj-$(CONFIG_TTY)		+= tty_io.o n_tty.o tty_ioctl.o tty_ldisc.o \

				   tty_buffer.o tty_port.o tty_mutex.o \

				   tty_ldsem.o tty_baudrate.o tty_jobctrl.o

				   tty_ldsem.o tty_baudrate.o tty_jobctrl.o \

				   n_null.o

obj-$(CONFIG_LEGACY_PTYS)	+= pty.o

obj-$(CONFIG_UNIX98_PTYS)	+= pty.o

obj-$(CONFIG_AUDIT)		+= tty_audit.o

drivers/tty/n_null.c

0 → 100644
+80 −0
Original line number Diff line number Diff line 
#include <linux/types.h>

#include <linux/errno.h>

#include <linux/tty.h>

#include <linux/module.h>



/*

 *  n_null.c - Null line discipline used in the failure path

 *

 *  Copyright (C) Intel 2017

 *

 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 *

 *  This program is free software; you can redistribute it and/or modify

 *  it under the terms of the GNU General Public License version 2

 *  as published by the Free Software Foundation.

 *

 *  This program is distributed in the hope that it will be useful,

 *  but WITHOUT ANY WARRANTY; without even the implied warranty of

 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

 *  GNU General Public License for more details.

 *

 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 */



static int n_null_open(struct tty_struct *tty)

{

	return 0;

}



static void n_null_close(struct tty_struct *tty)

{

}



static ssize_t n_null_read(struct tty_struct *tty, struct file *file,

			   unsigned char __user * buf, size_t nr)

{

	return -EOPNOTSUPP;

}



static ssize_t n_null_write(struct tty_struct *tty, struct file *file,

			    const unsigned char *buf, size_t nr)

{

	return -EOPNOTSUPP;

}



static void n_null_receivebuf(struct tty_struct *tty,

				 const unsigned char *cp, char *fp,

				 int cnt)

{

}



static struct tty_ldisc_ops null_ldisc = {

	.owner		=	THIS_MODULE,

	.magic		=	TTY_LDISC_MAGIC,

	.name		=	"n_null",

	.open		=	n_null_open,

	.close		=	n_null_close,

	.read		=	n_null_read,

	.write		=	n_null_write,

	.receive_buf	=	n_null_receivebuf

};



static int __init n_null_init(void)

{

	BUG_ON(tty_register_ldisc(N_NULL, &null_ldisc));

	return 0;

}



static void __exit n_null_exit(void)

{

	tty_unregister_ldisc(N_NULL);

}



module_init(n_null_init);

module_exit(n_null_exit);



MODULE_LICENSE("GPL");

MODULE_AUTHOR("Alan Cox");

MODULE_ALIAS_LDISC(N_NULL);

MODULE_DESCRIPTION("Null ldisc driver");

drivers/tty/tty_ldisc.c

+30 −14
Original line number Diff line number Diff line
@@ -491,6 +491,29 @@ static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld) 
	tty_ldisc_debug(tty, "%p: closed\n", ld);

}



/**

 *	tty_ldisc_failto	-	helper for ldisc failback

 *	@tty: tty to open the ldisc on

 *	@ld: ldisc we are trying to fail back to

 *

 *	Helper to try and recover a tty when switching back to the old

 *	ldisc fails and we need something attached.

 */



static int tty_ldisc_failto(struct tty_struct *tty, int ld)

{

	struct tty_ldisc *disc = tty_ldisc_get(tty, ld);

	int r;



	if (IS_ERR(disc))

		return PTR_ERR(disc);

	tty->ldisc = disc;

	tty_set_termios_ldisc(tty, ld);

	if ((r = tty_ldisc_open(tty, disc)) < 0)

		tty_ldisc_put(disc);

	return r;

}



/**

 *	tty_ldisc_restore	-	helper for tty ldisc change

 *	@tty: tty to recover
@@ -502,9 +525,6 @@ static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld) 


static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old)

{

	struct tty_ldisc *new_ldisc;

	int r;



	/* There is an outstanding reference here so this is safe */

	old = tty_ldisc_get(tty, old->ops->num);

	WARN_ON(IS_ERR(old));
@@ -512,17 +532,13 @@ static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old) 
	tty_set_termios_ldisc(tty, old->ops->num);

	if (tty_ldisc_open(tty, old) < 0) {

		tty_ldisc_put(old);

		/* This driver is always present */

		new_ldisc = tty_ldisc_get(tty, N_TTY);

		if (IS_ERR(new_ldisc))

			panic("n_tty: get");

		tty->ldisc = new_ldisc;

		tty_set_termios_ldisc(tty, N_TTY);

		r = tty_ldisc_open(tty, new_ldisc);

		if (r < 0)

			panic("Couldn't open N_TTY ldisc for "

			      "%s --- error %d.",

			      tty_name(tty), r);

		/* The traditional behaviour is to fall back to N_TTY, we

		   want to avoid falling back to N_NULL unless we have no

		   choice to avoid the risk of breaking anything */

		if (tty_ldisc_failto(tty, N_TTY) < 0 &&

		    tty_ldisc_failto(tty, N_NULL) < 0)

			panic("Couldn't open N_NULL ldisc for %s.",

			      tty_name(tty));

	}

}

include/uapi/linux/tty.h

+1 −0
Original line number Diff line number Diff line
@@ -36,5 +36,6 @@ 
#define N_TRACEROUTER	24	/* Trace data routing for MIPI P1149.7 */

#define N_NCI		25	/* NFC NCI UART */

#define N_SPEAKUP	26	/* Speakup communication with synths */

#define N_NULL		27	/* Null ldisc used for error handling */



#endif /* _UAPI_LINUX_TTY_H */