Re: [PATCH] tty: n_tty: fix KCSAN data-race in n_tty_flush_buffer / n_tty_lookahead_flow_ctrl

[Osama Abdelkader <osama.abdelkader@gmail.com>]

On Thu, Mar 12, 2026 at 03:21:03PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Feb 11, 2026 at 10:08:38PM +0100, Osama Abdelkader wrote:
> > n_tty_lookahead_flow_ctrl() accesses ldata->lookahead_count without
> > holding termios_rwsem, while reset_buffer_flags() in n_tty_flush_buffer()
> > resets it with exclusive termios_rwsem held. This causes a data race
> > reported by KCSAN when a PTY is closed while flush_to_ldisc is still
> > processing lookahead data.
> 
> A data race of what exactly?  lookahead_count?

yes, ldata->lookahead_count.

> 
> > Fix by taking termios_rwsem (read) in n_tty_lookahead_flow_ctrl(),
> > consistent with __receive_buf() which also modifies lookahead_count
> > under the read lock.
> 
> This feels wrong.  I would like to see a LOT of testing and validation
> that this is correct before being able to take this patch.  How was that
> done?
>

To clarify the reported race:

The race is on `ldata->lookahead_count` between:

1. `n_tty_lookahead_flow_ctrl()`, which does `lookahead_count += count`
   from the `flush_to_ldisc()` workqueue path:

   `flush_to_ldisc() -> lookahead_bufs() -> tty_port_default_lookahead_buf() -> n_tty_lookahead_flow_ctrl()`

2. `reset_buffer_flags()`, which does `lookahead_count = 0`
   from `n_tty_flush_buffer()` under write `termios_rwsem`
   (for example during PTY close / hangup).

`__receive_buf()` already accesses the same field under read `termios_rwsem`,
so this lookahead path was the remaining unlocked access to that state.

Because `n_tty_lookahead_flow_ctrl()` performs a read-modify-write (`+= count`),
`READ_ONCE()/WRITE_ONCE()` would still allow lost updates, so taking read
`termios_rwsem` there matches the existing writer-side protection.

I also ran the change with lockdep enabled and did not observe lockdep warnings
on PTY open/close stress.
 
> thanks,
> 
> greg k-h

Thanks,
Osama