tty: Fix race condition between __tty_buffer_request_room and flush_to_ldisc

The race was introduced while development of linux-3.11 by
e8437d7ecbc50198705331449367d401ebb3181f and
e9975fdec0138f1b2a85b9624e41660abd9865d4.
Originally it was found and reproduced on linux-3.12.15 and
linux-3.12.15-rt25, by sending 500 byte blocks with 115kbaud to the
target uart in a loop with 100 milliseconds delay.

In short:
1. The consumer flush_to_ldisc is on to remove the head tty_buffer.
2. The producer adds a number of bytes, so that a new tty_buffer must
be allocated and added by __tty_buffer_request_room.
3. The consumer removes the head tty_buffer element, without handling
newly committed data.

Detailed example:
* Initial buffer:
* Head, Tail -> 0: used=250; commit=250; read=240; next=NULL
* Consumer: ''flush_to_ldisc''
* consumed 10 Byte
* buffer:
* Head, Tail -> 0: used=250; commit=250; read=250; next=NULL
{{{
count = head->commit - head->read; // count = 0
if (!count) { // enter
// INTERRUPTED BY PRODUCER ->
if (head->next == NULL)
break;
buf->head = head->next;
tty_buffer_free(port, head);
continue;
}
}}}
* Producer: tty_insert_flip_... 10 bytes + tty_flip_buffer_push
* buffer:
* Head, Tail -> 0: used=250; commit=250; read=250; next=NULL
* added 6 bytes: head-element filled to maximum.
* buffer:
* Head, Tail -> 0: used=256; commit=250; read=250; next=NULL
* added 4 bytes: __tty_buffer_request_room is called
* buffer:
* Head -> 0: used=256; commit=256; read=250; next=1
* Tail -> 1: used=4; commit=0; read=250 next=NULL
* push (tty_flip_buffer_push)
* buffer:
* Head -> 0: used=256; commit=256; read=250; next=1
* Tail -> 1: used=4; commit=4; read=250 next=NULL
* Consumer
{{{
count = head->commit - head->read;
if (!count) {
// INTERRUPTED BY PRODUCER <-
if (head->next == NULL) // -> no break
break;
buf->head = head->next;
tty_buffer_free(port, head);
// ERROR: tty_buffer head freed -> 6 bytes lost
continue;
}
}}}

This patch reintroduces a spin_lock to protect this case. Perhaps later
a lock-less solution could be found.

Signed-off-by: Manfred Schlaegl <[e-mail]>
Cc: stable <[e-mail]> # 3.11
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/tty_buffer.c

···
 	if (change || left < size) {
 		/* This is the slow path - looking for new buffers to use */
 		if ((n = tty_buffer_alloc(port, size)) != NULL) {
+			unsigned long iflags;
+
 			n->flags = flags;
 			buf->tail = n;
+
+			spin_lock_irqsave(&buf->flush_lock, iflags);
 			b->commit = b->used;
-			smp_mb();
 			b->next = n;
+			spin_unlock_irqrestore(&buf->flush_lock, iflags);
+
 		} else if (change)
 			size = 0;
 		else
···
 	mutex_lock(&buf->lock);
 
 	while (1) {
+		unsigned long flags;
 		struct tty_buffer *head = buf->head;
 		int count;
 
···
 		if (atomic_read(&buf->priority))
 			break;
 
+		spin_lock_irqsave(&buf->flush_lock, flags);
 		count = head->commit - head->read;
 		if (!count) {
-			if (head->next == NULL)
+			if (head->next == NULL) {
+				spin_unlock_irqrestore(&buf->flush_lock, flags);
 				break;
+			}
 			buf->head = head->next;
+			spin_unlock_irqrestore(&buf->flush_lock, flags);
 			tty_buffer_free(port, head);
 			continue;
 		}
+		spin_unlock_irqrestore(&buf->flush_lock, flags);
 
 		count = receive_buf(tty, head, count);
 		if (!count)
···
 	struct tty_bufhead *buf = &port->buf;
 
 	mutex_init(&buf->lock);
+	spin_lock_init(&buf->flush_lock);
 	tty_buffer_reset(&buf->sentinel, 0);
 	buf->head = &buf->sentinel;
 	buf->tail = &buf->sentinel;

include/linux/tty.h

···
 	struct tty_buffer *head;	/* Queue head */
 	struct work_struct work;
 	struct mutex	   lock;
+	spinlock_t	   flush_lock;
 	atomic_t	   priority;
 	struct tty_buffer sentinel;
 	struct llist_head free;		/* Free queue head */