[Devel] [PATCH hci-8.1] ve/vtty: fix console attach failing with EBUSY while the CT closes the tty

[no-reply at sashiko.virtuozzo.com]

> From: Denis V. Lunev <den at 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 already
> means a console is attached, so userspace cannot tell a pair that is
> about to go from one that is taken, 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.
> 
> Return -EAGAIN for the dying pair. Nothing else on the
> VE_CONFIGURE_OPEN_TTY path returns -EAGAIN, so it can only mean "try
> again", which is what vzctl will do. An in-CT open of such a pair gets
> -EAGAIN from tty_reopen() and tty_open() retries it the same way.
> Retrying inside vtty_open_master() instead would have to drop tty_mutex
> for release_tty() to progress, and the global vtty context with it.
> 
> Let ve_vtty_stress accept -EAGAIN as an expected open error.
> 
> 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
> Signed-off-by: Denis V. Lunev <den at openvz.org>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.vzint.dev/#/patchset/20261029223502.1437160-1-den@openvz.org?part=1