[PATCH net 1/2] ppp: ppp_async: simplify tty disc_data access

[Qingfang Deng <qingfang.deng@linux.dev>]

tty_ldisc_hangup() invokes the hangup callback while holding only a read
lock on tty->ldisc_sem, so it can run concurrently with other line
discipline callbacks. This currently forces async PPP to maintain
separate lifetime protection around tty->disc_data.

Line discipline close is called under the write lock during hangup
processing. Remove the hangup callback and rely on close for teardown,
as done for SLIP by commit 23c53269f2ba ("slip: remove slip_hangup() to
fix use-after-free in slip_receive_buf()"). This serializes teardown
with all other line discipline operations.

disc_data_lock, refcount and completion are redundant with that
serialization. Remove them and access tty->disc_data directly.

This also eliminates a lockdep warning reported by syzbot. The warning
does not indicate a real deadlock because the write side runs only in
process context with hardirqs disabled.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+8e808eb853386f575d86@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/0000000000002fbad30611e25849@google.com/
Signed-off-by: Qingfang Deng <qingfang.deng@linux.dev>
---
 drivers/net/ppp/ppp_async.c | 82 ++++---------------------------------
 1 file changed, 7 insertions(+), 75 deletions(-)

diff --git a/drivers/net/ppp/ppp_async.c b/drivers/net/ppp/ppp_async.c
index 583426d06381..ea7fe9608ffd 100644
--- a/drivers/net/ppp/ppp_async.c
+++ b/drivers/net/ppp/ppp_async.c
@@ -63,8 +63,6 @@ struct asyncppp {
 
 	struct tasklet_struct tsk;
 
-	refcount_t	refcnt;
-	struct completion dead;
 	struct ppp_channel chan;	/* interface to generic ppp layer */
 	unsigned char	obuf[OBUFSIZE];
 };
@@ -114,38 +112,6 @@ static const struct ppp_channel_ops async_ops = {
  * Routines implementing the PPP line discipline.
  */
 
-/*
- * We have a potential race on dereferencing tty->disc_data,
- * because the tty layer provides no locking at all - thus one
- * cpu could be running ppp_asynctty_receive while another
- * calls ppp_asynctty_close, which zeroes tty->disc_data and
- * frees the memory that ppp_asynctty_receive is using.  The best
- * way to fix this is to use a rwlock in the tty struct, but for now
- * we use a single global rwlock for all ttys in ppp line discipline.
- *
- * FIXME: this is no longer true. The _close path for the ldisc is
- * now guaranteed to be sane.
- */
-static DEFINE_RWLOCK(disc_data_lock);
-
-static struct asyncppp *ap_get(struct tty_struct *tty)
-{
-	struct asyncppp *ap;
-
-	read_lock(&disc_data_lock);
-	ap = tty->disc_data;
-	if (ap != NULL)
-		refcount_inc(&ap->refcnt);
-	read_unlock(&disc_data_lock);
-	return ap;
-}
-
-static void ap_put(struct asyncppp *ap)
-{
-	if (refcount_dec_and_test(&ap->refcnt))
-		complete(&ap->dead);
-}
-
 /*
  * Called when a tty is put into PPP line discipline. Called in process
  * context.
@@ -180,9 +146,6 @@ ppp_asynctty_open(struct tty_struct *tty)
 	skb_queue_head_init(&ap->rqueue);
 	tasklet_setup(&ap->tsk, ppp_async_process);
 
-	refcount_set(&ap->refcnt, 1);
-	init_completion(&ap->dead);
-
 	ap->chan.private = ap;
 	ap->chan.ops = &async_ops;
 	ap->chan.mtu = PPP_MRU;
@@ -203,34 +166,18 @@ ppp_asynctty_open(struct tty_struct *tty)
 }
 
 /*
- * Called when the tty is put into another line discipline
- * or it hangs up.  We have to wait for any cpu currently
- * executing in any of the other ppp_asynctty_* routines to
- * finish before we can call ppp_unregister_channel and free
- * the asyncppp struct.  This routine must be called from
- * process context, not interrupt or softirq context.
+ * Called when the tty is put into another line discipline or it hangs up.
+ * This call is serialized against other ldisc functions.
  */
 static void
 ppp_asynctty_close(struct tty_struct *tty)
 {
-	struct asyncppp *ap;
+	struct asyncppp *ap = tty->disc_data;
 
-	write_lock_irq(&disc_data_lock);
-	ap = tty->disc_data;
-	tty->disc_data = NULL;
-	write_unlock_irq(&disc_data_lock);
 	if (!ap)
 		return;
 
-	/*
-	 * We have now ensured that nobody can start using ap from now
-	 * on, but we have to wait for all existing users to finish.
-	 * Note that ppp_unregister_channel ensures that no calls to
-	 * our channel ops (i.e. ppp_async_send/ioctl) are in progress
-	 * by the time it returns.
-	 */
-	if (!refcount_dec_and_test(&ap->refcnt))
-		wait_for_completion(&ap->dead);
+	tty->disc_data = NULL;
 	tasklet_kill(&ap->tsk);
 
 	ppp_unregister_channel(&ap->chan);
@@ -240,17 +187,6 @@ ppp_asynctty_close(struct tty_struct *tty)
 	kfree(ap);
 }
 
-/*
- * Called on tty hangup in process context.
- *
- * Wait for I/O to driver to complete and unregister PPP channel.
- * This is already done by the close routine, so just call that.
- */
-static void ppp_asynctty_hangup(struct tty_struct *tty)
-{
-	ppp_asynctty_close(tty);
-}
-
 /*
  * Read does nothing - no data is ever available this way.
  * Pppd reads and writes packets via /dev/ppp instead.
@@ -281,7 +217,7 @@ ppp_asynctty_write(struct tty_struct *tty, struct file *file, const u8 *buf,
 static int
 ppp_asynctty_ioctl(struct tty_struct *tty, unsigned int cmd, unsigned long arg)
 {
-	struct asyncppp *ap = ap_get(tty);
+	struct asyncppp *ap = tty->disc_data;
 	int err, val;
 	int __user *p = (int __user *)arg;
 
@@ -322,7 +258,6 @@ ppp_asynctty_ioctl(struct tty_struct *tty, unsigned int cmd, unsigned long arg)
 		err = tty_mode_ioctl(tty, cmd, arg);
 	}
 
-	ap_put(ap);
 	return err;
 }
 
@@ -331,7 +266,7 @@ static void
 ppp_asynctty_receive(struct tty_struct *tty, const u8 *buf, const u8 *cflags,
 		     size_t count)
 {
-	struct asyncppp *ap = ap_get(tty);
+	struct asyncppp *ap = tty->disc_data;
 	unsigned long flags;
 
 	if (!ap)
@@ -341,21 +276,19 @@ ppp_asynctty_receive(struct tty_struct *tty, const u8 *buf, const u8 *cflags,
 	spin_unlock_irqrestore(&ap->recv_lock, flags);
 	if (!skb_queue_empty(&ap->rqueue))
 		tasklet_schedule(&ap->tsk);
-	ap_put(ap);
 	tty_unthrottle(tty);
 }
 
 static void
 ppp_asynctty_wakeup(struct tty_struct *tty)
 {
-	struct asyncppp *ap = ap_get(tty);
+	struct asyncppp *ap = tty->disc_data;
 
 	clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 	if (!ap)
 		return;
 	set_bit(XMIT_WAKEUP, &ap->xmit_flags);
 	tasklet_schedule(&ap->tsk);
-	ap_put(ap);
 }
 
 
@@ -365,7 +298,6 @@ static struct tty_ldisc_ops ppp_ldisc = {
 	.name	= "ppp",
 	.open	= ppp_asynctty_open,
 	.close	= ppp_asynctty_close,
-	.hangup	= ppp_asynctty_hangup,
 	.read	= ppp_asynctty_read,
 	.write	= ppp_asynctty_write,
 	.ioctl	= ppp_asynctty_ioctl,
-- 
2.43.0