Re: [PATCH 2/4] tty: Hold tty_ldisc_lock() during tty_reopen()
From: Sergey Senozhatsky <hidden>
Date: 2018-08-29 04:34:39
Also in:
lkml
Hi, Cc-ing Benjamin on this. On (08/29/18 03:23), Dmitry Safonov wrote:
BUG: unable to handle kernel paging request at 0000000000002260 IP: [..] n_tty_receive_buf_common+0x5f/0x86d Workqueue: events_unbound flush_to_ldisc Call Trace: [..] n_tty_receive_buf2 [..] tty_ldisc_receive_buf [..] flush_to_ldisc [..] process_one_work [..] worker_thread [..] kthread [..] ret_from_fork
Seems that you are not the first one to hit this NULL deref.
I think, tty_ldisc_reinit() should be called with ldisc_sem hold for writing, which will protect any reader against line discipline changes.
Per https://lore.kernel.org/patchwork/patch/777220/ : Note that we noticed one path that called reinit without the ldisc lock : held for writing, we added that, but it didn't fix the problem. And I guess that Ben meant the same reinit path which you patched:
quoted hunk ↗ jump to hunk
@@ -1267,15 +1267,20 @@ static int tty_reopen(struct tty_struct *tty) if (test_bit(TTY_EXCLUSIVE, &tty->flags) && !capable(CAP_SYS_ADMIN)) return -EBUSY; - tty->count++; + retval = tty_ldisc_lock(tty, 5 * HZ); + if (retval) + return retval; + tty->count++; if (tty->ldisc) - return 0; + goto out_unlock; retval = tty_ldisc_reinit(tty, tty->termios.c_line); if (retval) tty->count--; +out_unlock: + tty_ldisc_unlock(tty); return retval; }
-ss