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 <manfred.schlaegl@gmx.at>
Cc: stable <stable@vger.kernel.org> # 3.11
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 15 insertions, 2 deletions

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 8ebd9f88a6f6..f1d30f6945af 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -255,11 +255,16 @@ static int __tty_buffer_request_room(struct tty_port *port, size_t size,
         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
@@ -443,6 +448,7 @@ static void flush_to_ldisc(struct work_struct *work)
         mutex_lock(&buf->lock);
 
         while (1) {
+                unsigned long flags;
                 struct tty_buffer *head = buf->head;
                 int count;
 
@@ -450,14 +456,19 @@ static void flush_to_ldisc(struct work_struct *work)
                 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)
@@ -512,6 +523,7 @@ void tty_buffer_init(struct tty_port *port)
         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;
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 1c3316a47d7e..036cccd80d9f 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -61,6 +61,7 @@ struct tty_bufhead {
         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 */