[3.13.y.z extended stable] Patch "tty: Fix lockless tty buffer race" has been added to staging queue

[Kamal Mostafa]

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

    tty: Fix lockless tty buffer race

to the linux-3.13.y-queue branch of the 3.13.y.z 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 3.13.11.3.

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.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From e6227bdf60685a26bad678cfc005acf9b18000a7 Mon Sep 17 00:00:00 2001
From: Peter Hurley <peter at hurleysoftware.com>
Date: Fri, 2 May 2014 10:56:12 -0400
Subject: tty: Fix lockless tty buffer race

commit 62a0d8d7c2b29f92850e4ee3c38e5dfd936e92b2 upstream.

Commit 6a20dbd6caa2358716136144bf524331d70b1e03,
"tty: Fix race condition between __tty_buffer_request_room and flush_to_ldisc"
correctly identifies an unsafe race condition between
__tty_buffer_request_room() and flush_to_ldisc(), where the consumer
flush_to_ldisc() prematurely advances the head before consuming the
last of the data committed. For example:

           CPU 0                     |            CPU 1
__tty_buffer_request_room            | flush_to_ldisc
  ...                                |   ...
                                     |   count = head->commit - head->read
  n = tty_buffer_alloc()             |
  b->commit = b->used                |
  b->next = n                        |
                                     |   if (!count)                /* T */
                                     |     if (head->next == NULL)  /* F */
                                     |     buf->head = head->next

In this case, buf->head has been advanced but head->commit may have
been updated with a new value.

Instead of reintroducing an unnecessary lock, fix the race locklessly.
Read the commit-next pair in the reverse order of writing, which guarantees
the commit value read is the latest value written if the head is
advancing.

Reported-by: Manfred Schlaegl <manfred.schlaegl at gmx.at>
Signed-off-by: Peter Hurley <peter at hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/tty/tty_buffer.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 2b52d80..4847fc5 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -248,7 +248,11 @@ int tty_buffer_request_room(struct tty_port *port, size_t size)
 		if ((n = tty_buffer_alloc(port, size)) != NULL) {
 			buf->tail = n;
 			b->commit = b->used;
-			smp_mb();
+			/* paired w/ barrier in flush_to_ldisc(); ensures the
+			 * latest commit value can be read before the head is
+			 * advanced to the next buffer
+			 */
+			smp_wmb();
 			b->next = n;
 		} else
 			size = left;
@@ -449,17 +453,24 @@ static void flush_to_ldisc(struct work_struct *work)

 	while (1) {
 		struct tty_buffer *head = buf->head;
+		struct tty_buffer *next;
 		int count;

 		/* Ldisc or user is trying to gain exclusive access */
 		if (atomic_read(&buf->priority))
 			break;

+		next = head->next;
+		/* paired w/ barrier in __tty_buffer_request_room();
+		 * ensures commit value read is not stale if the head
+		 * is advancing to the next buffer
+		 */
+		smp_rmb();
 		count = head->commit - head->read;
 		if (!count) {
-			if (head->next == NULL)
+			if (next == NULL)
 				break;
-			buf->head = head->next;
+			buf->head = next;
 			tty_buffer_free(port, head);
 			continue;
 		}
--
1.9.1