linux-kernel - [PATCH 3.18 099/150] n_tty: Fix read_buf race condition, increment read

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Message-Id: <20150114072100.356393055@linuxfoundation.org>
Date:	Tue, 13 Jan 2015 23:22:50 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org,
	Christian Riesch <christian.riesch@...cron.at>
Subject: [PATCH 3.18 099/150] n_tty: Fix read_buf race condition, increment read_head after pushing data

3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Christian Riesch <christian.riesch@...cron.at>

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@...cron.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/tty/n_tty.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -321,7 +321,8 @@ static void n_tty_check_unthrottle(struc

 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++;
 }

 /**

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