tty: Fix ldisc crash on reopened tty

peterhurley · gregkh · commit 71472fa9c52b · 2017-03-17T14:16:09.000+09:00
If the tty has been hungup, the ldisc instance may have been destroyed. Continued input to the tty will be ignored as long as the ldisc instance is not visible to the flush_to_ldisc kworker. However, when the tty is reopened and a new ldisc instance is created, the flush_to_ldisc kworker can obtain an ldisc reference before the new ldisc is completely initialized. This will likely crash: BUG: unable to handle kernel paging request at 0000000000002260 IP: [<ffffffff8152dc5d>] n_tty_receive_buf_common+0x6d/0xb80 PGD 2ab581067 PUD 290c11067 PMD 0 Oops: 0000 [#1] PREEMPT SMP Modules linked in: nls_iso8859_1 ip6table_filter [.....] CPU: 2 PID: 103 Comm: kworker/u16:1 Not tainted 4.6.0-rc7+wip-xeon+debug #rc7+wip Hardware name: Dell Inc. Precision WorkStation T5400 /0RW203, BIOS A11 04/30/2012 Workqueue: events_unbound flush_to_ldisc task: ffff8802ad16d100 ti: ffff8802ad31c000 task.ti: ffff8802ad31c000 RIP: 0010:[<ffffffff8152dc5d>] [<ffffffff8152dc5d>] n_tty_receive_buf_common+0x6d/0xb80 RSP: 0018:ffff8802ad31fc70 EFLAGS: 00010296 RAX: 0000000000000000 RBX: ffff8802aaddd800 RCX: 0000000000000001 RDX: 00000000ffffffff RSI: ffffffff810db48f RDI: 0000000000000246 RBP: ffff8802ad31fd08 R08: 0000000000000000 R09: 0000000000000001 R10: ffff8802aadddb28 R11: 0000000000000001 R12: ffff8800ba6da808 R13: ffff8802ad18be80 R14: ffff8800ba6da858 R15: ffff8800ba6da800 FS: 0000000000000000(0000) GS:ffff8802b0a00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000002260 CR3: 000000028ee5d000 CR4: 00000000000006e0 Stack: ffffffff81531219 ffff8802aadddab8 ffff8802aadddde0 ffff8802aadddd78 ffffffff00000001 ffff8800ba6da858 ffff8800ba6da860 ffff8802ad31fd30 ffffffff81885f78 ffffffff81531219 0000000000000000 0000000200000000 Call Trace: [<ffffffff81531219>] ? flush_to_ldisc+0x49/0xd0 [<ffffffff81885f78>] ? mutex_lock_nested+0x2c8/0x430 [<ffffffff81531219>] ? flush_to_ldisc+0x49/0xd0 [<ffffffff8152e784>] n_tty_receive_buf2+0x14/0x20 [<ffffffff81530cb2>] tty_ldisc_receive_buf+0x22/0x50 [<ffffffff8153128e>] flush_to_ldisc+0xbe/0xd0 [<ffffffff810a0ebd>] process_one_work+0x1ed/0x6e0 [<ffffffff810a0e3f>] ? process_one_work+0x16f/0x6e0 [<ffffffff810a13fe>] worker_thread+0x4e/0x490 [<ffffffff810a13b0>] ? process_one_work+0x6e0/0x6e0 [<ffffffff810a7ef2>] kthread+0xf2/0x110 [<ffffffff810ae68c>] ? preempt_count_sub+0x4c/0x80 [<ffffffff8188ab52>] ret_from_fork+0x22/0x50 [<ffffffff810a7e00>] ? kthread_create_on_node+0x220/0x220 Code: ff ff e8 27 a0 35 00 48 8d 83 78 05 00 00 c7 45 c0 00 00 00 00 48 89 45 80 48 8d 83 e0 05 00 00 48 89 85 78 ff ff ff 48 8b 45 b8 <48> 8b b8 60 22 00 00 48 8b 30 89 f8 8b 8b 88 04 00 00 29 f0 8d RIP [<ffffffff8152dc5d>] n_tty_receive_buf_common+0x6d/0xb80 RSP <ffff8802ad31fc70> CR2: 0000000000002260 Ensure the kworker cannot obtain the ldisc reference until the new ldisc is completely initialized. Fixes: 892d1fa ("tty: Destroy ldisc instance on hangup") Reported-by: Mikulas Patocka <mpatocka@redhat.com> Signed-off-by: Peter Hurley <peter@hurleysoftware.com> Signed-off-by: Michael Neuling <mikey@neuling.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
@@ -669,16 +669,17 @@ int tty_ldisc_reinit(struct tty_struct *tty, int disc)
 		tty_ldisc_put(tty->ldisc);
 	}
 
-	/* switch the line discipline */
-	tty->ldisc = ld;
 	tty_set_termios_ldisc(tty, disc);
-	retval = tty_ldisc_open(tty, tty->ldisc);
+	retval = tty_ldisc_open(tty, ld);
 	if (retval) {
 		if (!WARN_ON(disc == N_TTY)) {
-			tty_ldisc_put(tty->ldisc);
-			tty->ldisc = NULL;
+			tty_ldisc_put(ld);
+			ld = NULL;
 		}
 	}
+
+	/* switch the line discipline */
+	smp_store_release(&tty->ldisc, ld);
 	return retval;
 }
 

Original file line number	Diff line number	Diff line change
`@@ -669,16 +669,17 @@ int tty_ldisc_reinit(struct tty_struct *tty, int disc)`
`669`	`669`	`tty_ldisc_put(tty->ldisc);`
`670`	`670`	`}`
`671`	`671`
`672`		`- /* switch the line discipline */`
`673`		`- tty->ldisc = ld;`
`674`	`672`	`tty_set_termios_ldisc(tty, disc);`
`675`		`- retval = tty_ldisc_open(tty, tty->ldisc);`
	`673`	`+ retval = tty_ldisc_open(tty, ld);`
`676`	`674`	`if (retval) {`
`677`	`675`	`if (!WARN_ON(disc == N_TTY)) {`
`678`		`- tty_ldisc_put(tty->ldisc);`
`679`		`- tty->ldisc = NULL;`
	`676`	`+ tty_ldisc_put(ld);`
	`677`	`+ ld = NULL;`
`680`	`678`	`}`
`681`	`679`	`}`
	`680`	`+`
	`681`	`+ /* switch the line discipline */`
	`682`	`+ smp_store_release(&tty->ldisc, ld);`
`682`	`683`	`return retval;`
`683`	`684`	`}`
`684`	`685`