um: fix oopsable race in line_close()

tty->count is decremented only after ->close() had been called and
several tasks can hit it in parallel.  As the result, using tty->count
to check if you are the last one is broken.  We end up leaving line->tty
not reset to NULL and the next IRQ on that sucker will blow up trying to
dereference pointers from kfree'd struct tty.

Fix is obvious: we need to use a counter of our own.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 13 deletions

diff --git a/arch/um/drivers/line.c b/arch/um/drivers/line.c
index d51c404239a8..c5bff1ddeabc 100644
--- a/arch/um/drivers/line.c
+++ b/arch/um/drivers/line.c
@@ -399,8 +399,8 @@ int line_setup_irq(int fd, int input, int output, struct line *line, void *data)
  * is done under a spinlock.  Checking whether the device is in use is
  * line->tty->count > 1, also under the spinlock.
  *
- * tty->count serves to decide whether the device should be enabled or
- * disabled on the host.  If it's equal to 1, then we are doing the
+ * line->count serves to decide whether the device should be enabled or
+ * disabled on the host.  If it's equal to 0, then we are doing the
  * first open or last close.  Otherwise, open and close just return.
  */
 
@@ -414,16 +414,16 @@ int line_open(struct line *lines, struct tty_struct *tty)
                 goto out_unlock;
 
         err = 0;
-        if (tty->count > 1)
+        if (line->count++)
                 goto out_unlock;
 
-        spin_unlock(&line->count_lock);
-
+        BUG_ON(tty->driver_data);
         tty->driver_data = line;
         line->tty = tty;
 
+        spin_unlock(&line->count_lock);
         err = enable_chan(line);
-        if (err)
+        if (err) /* line_close() will be called by our caller */
                 return err;
 
         INIT_DELAYED_WORK(&line->task, line_timer_cb);
@@ -436,7 +436,7 @@ int line_open(struct line *lines, struct tty_struct *tty)
         chan_window_size(&line->chan_list, &tty->winsize.ws_row,
                          &tty->winsize.ws_col);
 
-        return err;
+        return 0;
 
 out_unlock:
         spin_unlock(&line->count_lock);
@@ -460,17 +460,16 @@ void line_close(struct tty_struct *tty, struct file * filp)
         flush_buffer(line);
 
         spin_lock(&line->count_lock);
-        if (!line->valid)
-                goto out_unlock;
+        BUG_ON(!line->valid);
 
-        if (tty->count > 1)
+        if (--line->count)
                 goto out_unlock;
 
-        spin_unlock(&line->count_lock);
-
         line->tty = NULL;
         tty->driver_data = NULL;
 
+        spin_unlock(&line->count_lock);
+
         if (line->sigio) {
                 unregister_winch(tty);
                 line->sigio = 0;
@@ -498,7 +497,7 @@ static int setup_one_line(struct line *lines, int n, char *init, int init_prio,
 
         spin_lock(&line->count_lock);
 
-        if (line->tty != NULL) {
+        if (line->count) {
                 *error_out = "Device is already open";
                 goto out;
         }