From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f179.google.com ([209.85.220.179]:35389 "EHLO mail-qk0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753360AbbEEKKI (ORCPT ); Tue, 5 May 2015 06:10:08 -0400 Received: by qkhg7 with SMTP id g7so101940484qkh.2 for ; Tue, 05 May 2015 03:10:08 -0700 (PDT) Message-ID: <554896FB.6020901@hurleysoftware.com> Date: Tue, 05 May 2015 06:10:03 -0400 From: Peter Hurley MIME-Version: 1.0 To: Greg Kroah-Hartman 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 References: <1428629912-22611-1-git-send-email-peter@hurleysoftware.com> <1428945874-30893-1-git-send-email-peter@hurleysoftware.com> <20150413181437.GA8144@kroah.com> In-Reply-To: <20150413181437.GA8144@kroah.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: stable-owner@vger.kernel.org List-ID: 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: # 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. Hi Greg, Can you push this to mainline for -rc3? Regards, Peter Hurley