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@xxxxxxxxxxxxxxx> # 3.19+
>> Reported-by: Andy Whitcroft <apw@xxxxxxxxxxxxx>
>> Reported-by: H.J. Lu <hjl.tools@xxxxxxxxx>
>> Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
>> ---
>
> 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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