[SRU X] [PATCH 0/5] Line discipline buffer flush/tty_reopen() race fix

[Guilherme G. Piccoli]

BugLink: https://bugs.launchpad.net/bugs/1791758

[Impact]

* Line discipline code is racy when we have buffer being flush while the
tty is being initialized or reinitialized. For the first problem, we have
an upstream patch since January 2018:
b027e2298bd5 ("tty: fix data race between tty_init_dev and flush of buf");
although it is not in Ubuntu kernel 4.4, only in kernels 4.15 and subsequent
ones.

* For the race between the buffer flush while tty is being reopened, we
have a patch that addresses this issue recently merged for 5.0-rc1:
83d817f41070 ("tty: Hold tty_ldisc_lock() during tty_reopen()").
No Ubuntu kernel currently contains this patch, hence we're hereby
submitting the SRU request. The upstream complete patch series for
this is in [0].

* The approach of both patches are similar - they rely in locking/semaphore
to prevent race conditions. Some additional patches are necessary to prevent
correlated issues, like preventing a potential deadlock due to bad
prioritization in servicing I/O over releasing tty_ldisc_lock() - refer to
c96cf923a98d ("tty: Don't block on IO when ldisc change is pending").
All the necessary fixes are grouped here in this SRU request.

* The symptom of the race condition between the buffer flush and the
tty reopen routine is a kernel crash with the following trace:


BUG: unable to handle kernel paging request at 0000000000002268
IP: [<addr>] n_tty_receive_buf_common+0x6a/0xae0
[...]
Call Trace:
[<addr>] ? kvm_sched_clock_read+0x1e/0x30
[<addr>] n_tty_receive_buf2+0x14/0x20
[<addr>] flush_to_ldisc+0xd5/0x120
[<addr>] process_one_work+0x156/0x400
[<addr>] worker_thread+0x11a/0x480
[...]


* A kernel crash was collected from an user, analysis is present in
comment #4 in LP #1791758.


[Test Case]

* It is not trivial to trigger this fault, but the usual recipe is to keep
accessing a machine through SSH (or keep killing getty when in IPMI serial
console) and in some way run commands before the terminal is ready in that
machine (like hacking some echo into ttySx or pts in an infinite loop).

* We have reports of users that could reproduce this issue in their
production environment, and with the patches present in this SRU request
the problem was fixed.


[Regression Potential]

* tty subsystem is highly central and patches in that area are always
delicate. For example, the upstream series [0] is a re-spin (V6) due to
a hard to reproduce issue reported in the PA-RISC architecture, which was
found in the V5 iteration [1] but was fixed by the patch c96cf923a98d,
present in this SRU request.

* The patchset [0] is present in tty-next tree since mid-November, and the
patch b027e2298bd5 is available upstream since January/2018 (it's available
in both Ubuntu kernels 4.15 and 4.18), so the overall likelihood of
regressions is low.

* These patches were sniff-tested for the 3 versions (4.4, 4.15 and 4.18)
and didn't show any issues.


[0] https://marc.info/?l=linux-kernel&m=154103190111795
[1] https://marc.info/?l=linux-kernel&m=153737852618183


Dmitry Safonov (4):
  tty: Drop tty->count on tty_reopen() failure
  tty: Hold tty_ldisc_lock() during tty_reopen()
  tty: Don't block on IO when ldisc change is pending
  tty: Simplify tty->count math in tty_reopen()

Gaurav Kohli (1):
  tty: fix data race between tty_init_dev and flush of buf

 drivers/tty/n_hdlc.c    |  4 ++--
 drivers/tty/n_r3964.c   |  2 +-
 drivers/tty/n_tty.c     |  8 ++++----
 drivers/tty/tty_io.c    | 20 +++++++++++++++++---
 drivers/tty/tty_ldisc.c | 11 +++++++++--
 include/linux/tty.h     |  9 +++++++++
 6 files changed, 42 insertions(+), 12 deletions(-)

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2.19.2