| From: Alan Cox <alan@llwyncelyn.cymru> |
| Date: Fri, 2 Jun 2017 13:49:30 +0100 |
| Subject: tty: handle the case where we cannot restore a line discipline |
| |
| commit 8a8dabf2dd68caff842d38057097c23bc514ea6e upstream. |
| |
| 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: Alan Cox <alan@linux.intel.com> |
| Reported-by: Dmitry Vyukov <dvyukov@google.com> |
| Tested-by: Dmitry Vyukov <dvyukov@google.com> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| [bwh: Backported to 3.16: adjust context] |
| Signed-off-by: Ben Hutchings <ben@decadent.org.uk> |
| --- |
| --- a/drivers/tty/Makefile |
| +++ b/drivers/tty/Makefile |
| @@ -1,5 +1,6 @@ |
| obj-$(CONFIG_TTY) += tty_io.o n_tty.o tty_ioctl.o tty_ldisc.o \ |
| - tty_buffer.o tty_port.o tty_mutex.o tty_ldsem.o |
| + tty_buffer.o tty_port.o tty_mutex.o \ |
| + tty_ldsem.o n_null.o |
| obj-$(CONFIG_LEGACY_PTYS) += pty.o |
| obj-$(CONFIG_UNIX98_PTYS) += pty.o |
| obj-$(CONFIG_AUDIT) += tty_audit.o |
| --- /dev/null |
| +++ b/drivers/tty/n_null.c |
| @@ -0,0 +1,80 @@ |
| +#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"); |
| --- a/drivers/tty/tty_ldisc.c |
| +++ b/drivers/tty/tty_ldisc.c |
| @@ -474,6 +474,29 @@ static void tty_ldisc_close(struct tty_s |
| } |
| |
| /** |
| + * 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 |
| * @old: previous ldisc |
| @@ -485,8 +508,6 @@ static void tty_ldisc_close(struct tty_s |
| static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old) |
| { |
| char buf[64]; |
| - 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); |
| @@ -495,17 +516,13 @@ static void tty_ldisc_restore(struct tty |
| 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, buf), 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, buf)); |
| } |
| } |
| |
| --- a/include/uapi/linux/tty.h |
| +++ b/include/uapi/linux/tty.h |
| @@ -34,5 +34,6 @@ |
| #define N_TI_WL 22 /* for TI's WL BT, FM, GPS combo chips */ |
| #define N_TRACESINK 23 /* Trace data routing for MIPI P1149.7 */ |
| #define N_TRACEROUTER 24 /* Trace data routing for MIPI P1149.7 */ |
| +#define N_NULL 27 /* Null ldisc used for error handling */ |
| |
| #endif /* _UAPI_LINUX_TTY_H */ |