[3.13.y-ckt stable] Patch "n_tty: Fix read_buf race condition, increment read_head after pushing data" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    n_tty: Fix read_buf race condition, increment read_head after pushing data

to the linux-3.13.y-queue branch of the 3.13.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11-ckt15.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From b04133d0e1b5b6d79a2675f7096ba3cf9a5a3dd1 Mon Sep 17 00:00:00 2001
From: Christian Riesch <christian.riesch at omicron.at>
Date: Thu, 13 Nov 2014 05:53:26 +0100
Subject: n_tty: Fix read_buf race condition, increment read_head after pushing
 data

commit 8bfbe2de769afda051c56aba5450391670e769fc upstream.

Commit 19e2ad6a09f0c06dbca19c98e5f4584269d913dd ("n_tty: Remove overflow
tests from receive_buf() path") moved the increment of read_head into
the arguments list of read_buf_addr(). Function calls represent a
sequence point in C. Therefore read_head is incremented before the
character c is placed in the buffer. Since the circular read buffer is
a lock-less design since commit 6d76bd2618535c581f1673047b8341fd291abc67
("n_tty: Make N_TTY ldisc receive path lockless"), this creates a race
condition that leads to communication errors.

This patch modifies the code to increment read_head _after_ the data
is placed in the buffer and thus fixes the race for non-SMP machines.
To fix the problem for SMP machines, memory barriers must be added in
a separate patch.

Signed-off-by: Christian Riesch <christian.riesch at omicron.at>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/tty/n_tty.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 86093e2..7bdaa14 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -319,7 +319,8 @@ static void n_tty_check_unthrottle(struct tty_struct *tty)

 static inline void put_tty_queue(unsigned char c, struct n_tty_data *ldata)
 {
-	*read_buf_addr(ldata, ldata->read_head++) = c;
+	*read_buf_addr(ldata, ldata->read_head) = c;
+	ldata->read_head++;
 }

 /**
--
1.9.1