fix serial buffer memory leak

Patch c5c34d4862e18ef07c1276d233507f540fb5a532 (tty: flush flip buffer on
ldisc input queue flush) introduces a race condition which can lead to memory
leaks.

The problem can be triggered when tcflush() is called when data are being
pushed to the line discipline driver by flush_to_ldisc().

flush_to_ldisc() releases tty->buf.lock when calling the line discipline
receive_buf function. At that poing tty_buffer_flush() kicks in and sets both
tty->buf.head and tty->buf.tail to NULL. When flush_to_ldisc() finishes, it
restores tty->buf.head but doesn't touch tty->buf.tail. This corrups the
buffer queue, and the next call to tty_buffer_request_room() will allocate a
new buffer and overwrite tty->buf.head. The previous buffer is then lost
forever without being released.

(Thanks to Laurent for the above text, for finding, disgnosing and reporting
the bug)

- Use tty->flags bits for the flush status.

- Wait for the flag to clear again before returning

- Fix the doc error noted

- Fix flush of empty queue leaving stale flushpending

[akpm@linux-foundation.org: cleanup]
Signed-off-by: Alan Cox <alan@redhat.com>
Acked-by: Paul Fulghum <paulkf@microgate.com>
Cc: Laurent Pinchart <laurentp@cse-semaphore.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 52 insertions, 6 deletions

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index de37ebc3a4cf..51ea93cab6c4 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -369,25 +369,54 @@ static void tty_buffer_free(struct tty_struct *tty, struct tty_buffer *b)
 }
 
 /**
- *      tty_buffer_flush                -       flush full tty buffers
+ *      __tty_buffer_flush              -       flush full tty buffers
  *      @tty: tty to flush
  *
- *      flush all the buffers containing receive data
+ *      flush all the buffers containing receive data. Caller must
+ *      hold the buffer lock and must have ensured no parallel flush to
+ *      ldisc is running.
  *
- *      Locking: none
+ *      Locking: Caller must hold tty->buf.lock
  */
 
-static void tty_buffer_flush(struct tty_struct *tty)
+static void __tty_buffer_flush(struct tty_struct *tty)
 {
         struct tty_buffer *thead;
-        unsigned long flags;
 
-        spin_lock_irqsave(&tty->buf.lock, flags);
         while((thead = tty->buf.head) != NULL) {
                 tty->buf.head = thead->next;
                 tty_buffer_free(tty, thead);
         }
         tty->buf.tail = NULL;
+}
+
+/**
+ *      tty_buffer_flush                -       flush full tty buffers
+ *      @tty: tty to flush
+ *
+ *      flush all the buffers containing receive data. If the buffer is
+ *      being processed by flush_to_ldisc then we defer the processing
+ *      to that function
+ *
+ *      Locking: none
+ */
+
+static void tty_buffer_flush(struct tty_struct *tty)
+{
+        unsigned long flags;
+        spin_lock_irqsave(&tty->buf.lock, flags);
+
+        /* If the data is being pushed to the tty layer then we can't
+           process it here. Instead set a flag and the flush_to_ldisc
+           path will process the flush request before it exits */
+        if (test_bit(TTY_FLUSHING, &tty->flags)) {
+                set_bit(TTY_FLUSHPENDING, &tty->flags);
+                spin_unlock_irqrestore(&tty->buf.lock, flags);
+                wait_event(tty->read_wait,
+                                test_bit(TTY_FLUSHPENDING, &tty->flags) == 0);
+                return;
+        } else
+                __tty_buffer_flush(tty);
         spin_unlock_irqrestore(&tty->buf.lock, flags);
 }
 
@@ -3594,6 +3623,7 @@ static void flush_to_ldisc(struct work_struct *work)
                 return;
 
         spin_lock_irqsave(&tty->buf.lock, flags);
+        set_bit(TTY_FLUSHING, &tty->flags);     /* So we know a flush is running */
         head = tty->buf.head;
         if (head != NULL) {
                 tty->buf.head = NULL;
@@ -3607,6 +3637,11 @@ static void flush_to_ldisc(struct work_struct *work)
                                 tty_buffer_free(tty, tbuf);
                                 continue;
                         }
+                        /* Ldisc or user is trying to flush the buffers
+                           we are feeding to the ldisc, stop feeding the
+                           line discipline as we want to empty the queue */
+                        if (test_bit(TTY_FLUSHPENDING, &tty->flags))
+                                break;
                         if (!tty->receive_room) {
                                 schedule_delayed_work(&tty->buf.work, 1);
                                 break;
@@ -3620,8 +3655,17 @@ static void flush_to_ldisc(struct work_struct *work)
                         disc->receive_buf(tty, char_buf, flag_buf, count);
                         spin_lock_irqsave(&tty->buf.lock, flags);
                 }
+                /* Restore the queue head */
                 tty->buf.head = head;
         }
+        /* We may have a deferred request to flush the input buffer,
+           if so pull the chain under the lock and empty the queue */
+        if (test_bit(TTY_FLUSHPENDING, &tty->flags)) {
+                __tty_buffer_flush(tty);
+                clear_bit(TTY_FLUSHPENDING, &tty->flags);
+                wake_up(&tty->read_wait);
+        }
+        clear_bit(TTY_FLUSHING, &tty->flags);
         spin_unlock_irqrestore(&tty->buf.lock, flags);
 
         tty_ldisc_deref(disc);
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 691a1748d9d2..6570719eafdf 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -274,6 +274,8 @@ struct tty_struct {
 #define TTY_PTY_LOCK            16      /* pty private */
 #define TTY_NO_WRITE_SPLIT      17      /* Preserve write boundaries to driver */
 #define TTY_HUPPED              18      /* Post driver->hangup() */
+#define TTY_FLUSHING            19      /* Flushing to ldisc in progress */
+#define TTY_FLUSHPENDING        20      /* Queued buffer flush pending */
 
 #define TTY_WRITE_FLUSH(tty) tty_write_flush((tty))