From d8d4d1cf40d541a5d7cc3b15d57e42d0815c7d53 Mon Sep 17 00:00:00 2001 From: Qingfang Deng Date: Fri, 28 Aug 2026 15:32:37 +0800 Subject: [PATCH] ppp: ppp_synctty: simplify tty disc_data access Apply the same simplification as the preceding ppp_async change. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Reported-by: syzbot+b503105c2410c3433459@syzkaller.appspotmail.com Closes: https://syzbot.org/bug?extid=b503105c2410c3433459 Signed-off-by: Qingfang Deng Reviewed-by: Eric Dumazet Link: https://patch.msgid.link/20260828073245.126804-2-qingfang.deng@linux.dev Signed-off-by: Jakub Kicinski --- drivers/net/ppp/ppp_synctty.c | 83 +++-------------------------------- 1 file changed, 7 insertions(+), 76 deletions(-) diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c index 0b1bd1635c39..f87d43faeeab 100644 --- a/drivers/net/ppp/ppp_synctty.c +++ b/drivers/net/ppp/ppp_synctty.c @@ -38,11 +38,9 @@ #include #include #include -#include #include #include #include -#include #include #include @@ -67,8 +65,6 @@ struct syncppp { struct tasklet_struct tsk; - refcount_t refcnt; - struct completion dead_cmp; struct ppp_channel chan; /* interface to generic ppp layer */ }; @@ -116,37 +112,6 @@ ppp_print_buffer (const char *name, const __u8 *buf, int count) * Routines implementing the synchronous 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_synctty_receive while another - * calls ppp_synctty_close, which zeroes tty->disc_data and - * frees the memory that ppp_synctty_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: Fixed in tty_io nowadays. - */ -static DEFINE_RWLOCK(disc_data_lock); - -static struct syncppp *sp_get(struct tty_struct *tty) -{ - struct syncppp *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 sp_put(struct syncppp *ap) -{ - if (refcount_dec_and_test(&ap->refcnt)) - complete(&ap->dead_cmp); -} - /* * Called when a tty is put into sync-PPP line discipline. */ @@ -177,9 +142,6 @@ ppp_sync_open(struct tty_struct *tty) skb_queue_head_init(&ap->rqueue); tasklet_setup(&ap->tsk, ppp_sync_process); - refcount_set(&ap->refcnt, 1); - init_completion(&ap->dead_cmp); - ap->chan.private = ap; ap->chan.ops = &sync_ops; ap->chan.mtu = PPP_MRU; @@ -201,34 +163,18 @@ ppp_sync_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_synctty_* routines to - * finish before we can call ppp_unregister_channel and free - * the syncppp 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_sync_close(struct tty_struct *tty) { - struct syncppp *ap; + struct syncppp *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_sync_send/ioctl) are in progress - * by the time it returns. - */ - if (!refcount_dec_and_test(&ap->refcnt)) - wait_for_completion(&ap->dead_cmp); + tty->disc_data = NULL; tasklet_kill(&ap->tsk); ppp_unregister_channel(&ap->chan); @@ -237,17 +183,6 @@ ppp_sync_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_sync_hangup(struct tty_struct *tty) -{ - ppp_sync_close(tty); -} - /* * Read does nothing - no data is ever available this way. * Pppd reads and writes packets via /dev/ppp instead. @@ -273,7 +208,7 @@ ppp_sync_write(struct tty_struct *tty, struct file *file, const u8 *buf, static int ppp_synctty_ioctl(struct tty_struct *tty, unsigned int cmd, unsigned long arg) { - struct syncppp *ap = sp_get(tty); + struct syncppp *ap = tty->disc_data; int __user *p = (int __user *)arg; int err, val; @@ -314,7 +249,6 @@ ppp_synctty_ioctl(struct tty_struct *tty, unsigned int cmd, unsigned long arg) break; } - sp_put(ap); return err; } @@ -323,7 +257,7 @@ static void ppp_sync_receive(struct tty_struct *tty, const u8 *buf, const u8 *cflags, size_t count) { - struct syncppp *ap = sp_get(tty); + struct syncppp *ap = tty->disc_data; unsigned long flags; if (!ap) @@ -333,21 +267,19 @@ ppp_sync_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); - sp_put(ap); tty_unthrottle(tty); } static void ppp_sync_wakeup(struct tty_struct *tty) { - struct syncppp *ap = sp_get(tty); + struct syncppp *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); - sp_put(ap); } @@ -357,7 +289,6 @@ static struct tty_ldisc_ops ppp_sync_ldisc = { .name = "pppsync", .open = ppp_sync_open, .close = ppp_sync_close, - .hangup = ppp_sync_hangup, .read = ppp_sync_read, .write = ppp_sync_write, .ioctl = ppp_synctty_ioctl,