[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>

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index a23816d3e9a1684794c8e5a8f1cc0cce26fb61d8..e9bba94fc898388ff09bd8a9e2116442e07ca104 100644 (file)
--- 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;