[Devel] [PATCH hci-8.1 2/2] ve/vtty: wait for a closing CT tty instead of failing the console attach

["Denis V. Lunev" <den@openvz.org>]

From: Denis V. Lunev <den@openvz.org>

Since commit 23abb203a41a ("ve/vtty: fix use-after-free on concurrent
tty close and reopen") vtty_open_master() refuses a pair whose slave
count has dropped to zero: the last CT-side close has committed to the
final close, and the pair stays in the vtty map only until
release_tty() runs. The refusal is -EBUSY, the errno which means a
console is attached, and vzctl gives up:

  Error opening CT tty: Device or resource busy

The window opens whenever the last process in the CT closes its tty:
getty respawn, login with autologin, SAK. "vzctl console" run right
after "vzctl start --wait" hits it.

Wait for the closer instead, as tty_open() does for the same window
since commit 7f22f6c935cd ("tty: Retry failed reopen if tty teardown
in-progress"): drop the slave lock, the vtty context and tty_mutex,
reschedule and look the pair up again. The fd and the file stay
allocated across the retry. No userspace change is needed, and -EBUSY
again means only that a console is attached.

The loop ends. Both counts are zero only after tty_release() has taken
the final close, and the closer then only needs tty_mutex in
tty_release_struct() to reach vtty_shutdown(), which removes the pair
from the vtty map. The next lookup misses and allocates a fresh pair.
The opener spins on schedule() while the closer flushes the tty works,
as tty_open() does. A pending signal ends the wait with -ERESTARTSYS,
so the ioctl restarts or fails with EINTR.

Fixes: 23abb203a41a ("ve/vtty: fix use-after-free on concurrent tty close and reopen")

https://virtuozzo.atlassian.net/browse/VSTOR-152365
Feature: tty: virtual Container console
Suggested-by: Konstantin Khorenko <khorenko@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
---
 drivers/tty/pty.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index 14d46bb9c293..6415456dca13 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -1054,6 +1054,7 @@ int vtty_open_master(envid_t veid, int idx)
 	 * else would access it while we're holding
 	 * the lock.
 	 */
+retry:
 	mutex_lock(&tty_mutex);
 	vtty_set_context(veid);
 
@@ -1100,15 +1101,23 @@ int vtty_open_master(envid_t veid, int idx)
 	 * user is gone and tty_release() has already committed to the final
 	 * close, release_tty() is only waiting for tty_mutex to free it.
 	 * Never hand out a reference to a tty which is about to be freed,
-	 * report -EBUSY so that the next attempt allocates a fresh pair.
+	 * let the closer through and retry, the next lookup then misses
+	 * and allocates a fresh pair. -EBUSY above means a console is
+	 * really attached.
 	 *
 	 * A pair we've just created above is exempt: it is not reachable by
 	 * anyone else yet and its zero count is merely the initial state,
 	 * vtty_install() gives its reference away to the master peer below.
 	 */
 	if (!created && slave->count == 0) {
-		ret = -EBUSY;
-		goto err_unlock;
+		ret = -ERESTARTSYS;
+		if (signal_pending(current))
+			goto err_unlock;
+		tty_unlock(slave);
+		vtty_drop_context();
+		mutex_unlock(&tty_mutex);
+		schedule();
+		goto retry;
 	}
 
 	/*
-- 
2.53.0

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