linux-kernel - Re: [PATCH v3] pty: Fix input race when closing

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Date:	Mon, 13 Apr 2015 20:14:37 +0200
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Jiri Slaby <jslaby@...e.cz>, Alan Cox <alan@...ux.intel.com>,
	"H.J. Lu" <hjl.tools@...il.com>, Josh Boyer <jwboyer@...hat.com>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v3] pty: Fix input race when closing

On Mon, Apr 13, 2015 at 01:24:34PM -0400, Peter Hurley wrote:
> A read() from a pty master may mistakenly indicate EOF (errno == -EIO)
> after the pty slave has closed, even though input data remains to be read.
> For example,
> 
>        pty slave       |        input worker        |    pty master
>                        |                            |
>                        |                            |   n_tty_read()
> pty_write()            |                            |     input avail? no
>   add data             |                            |     sleep
>   schedule worker  --->|                            |     .
>                        |---> flush_to_ldisc()       |     .
> pty_close()            |       fill read buffer     |     .
>   wait for worker      |       wakeup reader    --->|     .
>                        |       read buffer full?    |---> input avail ? yes
>                        |<---   yes - exit worker    |     copy 4096 bytes to user
>   TTY_OTHER_CLOSED <---|                            |<--- kick worker
>                        |                            |
> 
> 		                **** New read() before worker starts ****
> 
>                        |                            |   n_tty_read()
>                        |                            |     input avail? no
>                        |                            |     TTY_OTHER_CLOSED? yes
>                        |                            |     return -EIO
> 
> Several conditions are required to trigger this race:
> 1. the ldisc read buffer must become full so the input worker exits
> 2. the read() count parameter must be >= 4096 so the ldisc read buffer
>    is empty
> 3. the subsequent read() occurs before the kicked worker has processed
>    more input
> 
> However, the underlying cause of the race is that data is pipelined, while
> tty state is not; ie., data already written by the pty slave end is not
> yet visible to the pty master end, but state changes by the pty slave end
> are visible to the pty master end immediately.
> 
> Pipeline the TTY_OTHER_CLOSED state through input worker to the reader.
> 1. Introduce TTY_OTHER_DONE which is set by the input worker when
>    TTY_OTHER_CLOSED is set and either the input buffers are flushed or
>    input processing has completed. Readers/polls are woken when
>    TTY_OTHER_DONE is set.
> 2. Reader/poll checks TTY_OTHER_DONE instead of TTY_OTHER_CLOSED.
> 3. A new input worker is started from pty_close() after setting
>    TTY_OTHER_CLOSED, which ensures the TTY_OTHER_DONE state will be
>    set if the last input worker is already finished (or just about to
>    exit).
> 
> Remove tty_flush_to_ldisc(); no in-tree callers.
> 
> Fixes: 52bce7f8d4fc ("pty, n_tty: Simplify input processing on final close")
> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=96311
> BugLink: http://bugs.launchpad.net/bugs/1429756
> Cc: <stable@...r.kernel.org> # 3.19+
> Reported-by: Andy Whitcroft <apw@...onical.com>
> Reported-by: H.J. Lu <hjl.tools@...il.com>
> Signed-off-by: Peter Hurley <peter@...leysoftware.com>
> ---

Thanks for this, it's a bit late at the moment for 4.1-rc1, but I'll get
it queued up after that.

thanks,

greg k-h
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