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Commit da965822 authored by Paul Fulghum's avatar Paul Fulghum Committed by Linus Torvalds
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[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: default avatarPaul Fulghum <paulkf@microgate.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 16bf1348

drivers/char/tty_io.c

+3 −4
Original line number Diff line number Diff line
@@ -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;