Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754479AbbDMSOq (ORCPT ); Mon, 13 Apr 2015 14:14:46 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:55915 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751524AbbDMSOo (ORCPT ); Mon, 13 Apr 2015 14:14:44 -0400 Date: Mon, 13 Apr 2015 20:14:37 +0200 From: Greg Kroah-Hartman To: Peter Hurley Cc: Jiri Slaby , Alan Cox , "H.J. Lu" , Josh Boyer , linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH v3] pty: Fix input race when closing Message-ID: <20150413181437.GA8144@kroah.com> References: <1428629912-22611-1-git-send-email-peter@hurleysoftware.com> <1428945874-30893-1-git-send-email-peter@hurleysoftware.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1428945874-30893-1-git-send-email-peter@hurleysoftware.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3468 Lines: 71 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: # 3.19+ > Reported-by: Andy Whitcroft > Reported-by: H.J. Lu > Signed-off-by: Peter Hurley > --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/