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tty: Fix recursive deadlock in tty_perform_flush()

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tty_perform_flush() can deadlock when called while holding
a line discipline reference. By definition, all ldisc drivers
hold a ldisc reference, so calls originating from ldisc drivers
must not block for a ldisc reference.

The deadlock can occur when:
  CPU 0                    |  CPU 1
                           |
tty_ldisc_ref(tty)         |
....                       | <line discipline halted>
tty_ldisc_ref_wait(tty)    |
                           |

CPU 0 cannot progess because it cannot obtain an ldisc reference
with the line discipline has been halted (thus no new references
are granted).
CPU 1 cannot progress because an outstanding ldisc reference
has not been released.

An in-tree call-tree audit of tty_perform_flush() [1] shows 5
ldisc drivers calling tty_perform_flush() indirectly via
n_tty_ioctl_helper() and 2 ldisc drivers calling directly.
A single tty driver safely uses the function.

[1]
Recursive usage:

/* These functions are line discipline ioctls and thus
 * recursive wrt line discipline references */

tty_perform_flush() - ./drivers/tty/tty_ioctl.c
    n_tty_ioctl_helper()
        hci_uart_tty_ioctl(default) - drivers/bluetooth/hci_ldisc.c (N_HCI)
        n_hdlc_tty_ioctl(default) - drivers/tty/n_hdlc.c (N_HDLC)
        gsmld_ioctl(default) - drivers/tty/n_gsm.c (N_GSM0710)
        n_tty_ioctl(default) - drivers/tty/n_tty.c (N_TTY)
        gigaset_tty_ioctl(default) - drivers/isdn/gigaset/ser-gigaset.c (N_GIGASET_M101)
    ppp_synctty_ioctl(TCFLSH) - drivers/net/ppp/pps_synctty.c
    ppp_asynctty_ioctl(TCFLSH) - drivers/net/ppp/ppp_async.c

Non-recursive use:

tty_perform_flush() - drivers/tty/tty_ioctl.c
    ipw_ioctl(TCFLSH) - drivers/tty/ipwireless/tty.c
       /* This function is a tty i/o ioctl method, which
        * is invoked by tty_ioctl() */

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Peter Hurley 2013-03-11 16:44:45 -04:00 committed by Greg Kroah-Hartman
parent be3971166d
commit e7f3880cd9
3 changed files with 22 additions and 12 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

2
drivers/net/ppp/ppp_async.c
View file

@ -314,7 +314,7 @@ ppp_asynctty_ioctl(struct tty_struct *tty, struct file *file,
/* flush our buffers and the serial port's buffer */
if (arg == TCIOFLUSH || arg == TCOFLUSH)
ppp_async_flush_output(ap);
err = tty_perform_flush(tty, arg);
err = n_tty_ioctl_helper(tty, file, cmd, arg);
break;
case FIONREAD:

2
drivers/net/ppp/ppp_synctty.c
View file

@ -355,7 +355,7 @@ ppp_synctty_ioctl(struct tty_struct *tty, struct file *file,
/* flush our buffers and the serial port's buffer */
if (arg == TCIOFLUSH || arg == TCOFLUSH)
ppp_sync_flush_output(ap);
err = tty_perform_flush(tty, arg);
err = n_tty_ioctl_helper(tty, file, cmd, arg);
break;
case FIONREAD:

30
drivers/tty/tty_ioctl.c
View file

@ -1122,14 +1122,12 @@ int tty_mode_ioctl(struct tty_struct *tty, struct file *file,
}
EXPORT_SYMBOL_GPL(tty_mode_ioctl);
int tty_perform_flush(struct tty_struct *tty, unsigned long arg)
{
struct tty_ldisc *ld;
int retval = tty_check_change(tty);
if (retval)
return retval;
ld = tty_ldisc_ref_wait(tty);
/* Caller guarantees ldisc reference is held */
static int __tty_perform_flush(struct tty_struct *tty, unsigned long arg)
{
struct tty_ldisc *ld = tty->ldisc;
switch (arg) {
case TCIFLUSH:
if (ld && ld->ops->flush_buffer) {
@ -1147,12 +1145,24 @@ int tty_perform_flush(struct tty_struct *tty, unsigned long arg)
tty_driver_flush_buffer(tty);
break;
default:
tty_ldisc_deref(ld);
return -EINVAL;
}
tty_ldisc_deref(ld);
return 0;
}
int tty_perform_flush(struct tty_struct *tty, unsigned long arg)
{
struct tty_ldisc *ld;
int retval = tty_check_change(tty);
if (retval)
return retval;
ld = tty_ldisc_ref_wait(tty);
retval = __tty_perform_flush(tty, arg);
if (ld)
tty_ldisc_deref(ld);
return retval;
}
EXPORT_SYMBOL_GPL(tty_perform_flush);
int n_tty_ioctl_helper(struct tty_struct *tty, struct file *file,
@ -1191,7 +1201,7 @@ int n_tty_ioctl_helper(struct tty_struct *tty, struct file *file,
}
return 0;
case TCFLSH:
return tty_perform_flush(tty, arg);
return __tty_perform_flush(tty, arg);
default:
/* Try the mode commands */
return tty_mode_ioctl(tty, file, cmd, arg);