Re: [PATCH v3] pty: Fix input race when closing
From: Greg Kroah-Hartman
Date: Mon Apr 13 2015 - 14:14:48 EST
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
> 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.
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