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

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Date:	Tue, 05 May 2015 06:10:03 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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 04/13/2015 02:14 PM, Greg Kroah-Hartman wrote:
> 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.

Hi Greg,

Can you push this to mainline for -rc3?

Regards,
Peter Hurley

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