[PATCH] tty reference count fix

Fix hole where tty structure can be released when reference count is non
zero.  Existing code can sleep without tty_sem protection between deciding
to release the tty structure (setting local variables tty_closing and
otty_closing) and setting TTY_CLOSING to prevent further opens.  An open
can occur during this interval causing release_dev() to free the tty
structure while it is still referenced.

This should fix bugzilla.kernel.org [Bug 6041] New: Unable to handle kernel
paging request

In Bug 6041, tty_open() oopes on accessing the tty structure it has
successfully claimed.  Bug was on SMP machine with the same tty being
opened and closed by multiple processes, and DEBUG_PAGEALLOC enabled.

Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 3 insertions, 4 deletions

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index a23816d3e9..e9bba94fc8 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -1841,7 +1841,6 @@ static void release_dev(struct file * filp)
                 tty_closing = tty->count <= 1;
                 o_tty_closing = o_tty &&
                         (o_tty->count <= (pty_master ? 1 : 0));
-                up(&tty_sem);
                 do_sleep = 0;
 
                 if (tty_closing) {
@@ -1869,6 +1868,7 @@ static void release_dev(struct file * filp)
 
                 printk(KERN_WARNING "release_dev: %s: read/write wait queue "
                                     "active!\n", tty_name(tty, buf));
+                up(&tty_sem);
                 schedule();
         }       
 
@@ -1877,8 +1877,6 @@ static void release_dev(struct file * filp)
          * both sides, and we've completed the last operation that could 
          * block, so it's safe to proceed with closing.
          */
-         
-        down(&tty_sem);
         if (pty_master) {
                 if (--o_tty->count < 0) {
                         printk(KERN_WARNING "release_dev: bad pty slave count "
@@ -1892,7 +1890,6 @@ static void release_dev(struct file * filp)
                        tty->count, tty_name(tty, buf));
                 tty->count = 0;
         }
-        up(&tty_sem);
         
         /*
          * We've decremented tty->count, so we need to remove this file
@@ -1937,6 +1934,8 @@ static void release_dev(struct file * filp)
                 read_unlock(&tasklist_lock);
         }
 
+        up(&tty_sem);
+
         /* check whether both sides are closing ... */
         if (!tty_closing || (o_tty && !o_tty_closing))
                 return;