[PATCH -next 0/9] tty: Fix buffer work access-after-free

From: Peter Hurley
Date: Tue Dec 04 2012 - 02:11:51 EST

This patch series addresses the causes of flush_to_ldisc accessing
the tty after freeing.

The most common cause stems from the n_tty_close() path spuriously
scheduling buffer work, when the ldisc has already been halted.
This is fixed in 'tty: Don't reschedule buffer work while closing'

The other causes have a central theme: incorrect order-of-operations
when halting a line discipline. In general, to prevent buffer work
from being scheduled requires:
1. Disallowing further ldisc references
2. Waiting for all existing ldisc references to be released
3. Cancelling existing buffer work
If the wait takes place _after_ cancellation, then new work
can be scheduled by existing holder(s) of ldisc reference(s). That's

Halting the line discipline is performed when,
- hanging up the tty (tty_ldisc_hangup())
- TIOCSETD ioctl (tty_set_ldisc())
- finally closing the tty (pair) (tty_ldisc_release())

Concurrent halts are governed by the following requirements:
1. tty_ldisc_release is not concurrent with the other two and so
does not need lock or state protection during the ldiscs halt.
2. Accesses through tty->ldisc must be protected by the ldisc_mutex.
The wait operation checks the user count (ldisc references)
in tty->ldisc->users.
3. tty_set_ldisc() reschedules buffer work that was pending when
the ldiscs were halted. That must be an atomic operation in
conjunction with re-enabling the ldisc -- which necessitates
locking concurrent halts (tty_ldisc_release is exempt here)
4. The legacy mutex cannot be held while waiting for ldisc
reference(s) release -or- for cancelling buffer work.
5. Because of #4, the legacy mutex must be dropped prior to or
during the halt. Which means reacquiring after the halt. But
to preserve lock order the ldisc_mutex must be dropped and
reacquired after reacquiring the legacy mutex.
6. Because of #5, the ldisc state may have changed while the
ldisc mutex was dropped.

Note: this series does not include the lock correction initially
reported on by Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> here
https://lkml.org/lkml/2012/11/21/267 . I commented on the latest
version here https://lkml.org/lkml/2012/12/3/362

This series also does not include Jiri's debug patch here
https://lkml.org/lkml/2012/11/2/278 for identifying which itty cleanup
path is used. I think it may be helpful to include this in -next.

Peter Hurley (9):
tty: WARN if buffer work racing with tty free
tty: Add diagnostic for halted line discipline
tty: Don't reschedule buffer work while closing
tty: Refactor wait for ldisc refs out of tty_ldisc_hangup()
tty: Remove unnecessary re-test of ldisc ref count
tty: Fix ldisc halt sequence on hangup
tty: Strengthen no-subsequent-use guarantee of tty_ldisc_halt()
tty: Remove unnecessary buffer work flush
tty: Halt both ldiscs concurrently

drivers/tty/n_tty.c | 8 ++-
drivers/tty/tty_buffer.c | 3 +
drivers/tty/tty_io.c | 2 +
drivers/tty/tty_ldisc.c | 171 +++++++++++++++++++++++++++++------------------
include/linux/tty.h | 1 +
5 files changed, 119 insertions(+), 66 deletions(-)


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/