[PATCH] tty: vt: check for atomic context in con_write()

[Tetsuo Handa]

syzbot is reporting sleep in atomic context, for gsmld_write() is calling
con_write() with spinlock held and IRQs disabled.

Since include/linux/tty_ldisc.h says that "struct tty_ldisc_ops"->write
(e.g. gsmld_write()) is allowed to sleep and include/linux/tty_driver.h
says that "struct tty_operations"->write (e.g. con_write()) is not
allowed to sleep, we should handle this problem on the con_write() side.

It seems that "Andrew Morton: console locking merge" in 2.4.10-pre11 added
in_interrupt() check to do_con_write()/con_put_char()/con_flush_chars()
in order to handle exceptional caller.

Since include/linux/preempt.h says that in_atomic() cannot know about held
spinlocks in non-preemptible kernels, but gsmld_write() is calling
con_write() with IRQs disabled, we can add irqs_disabled() check to
do_con_write()/con_flush_chars() in order to handle this case. Though,
I'm not sure whether returning the bytes to write is appropriate behavior
when do_con_write() can't work...

Reported-by: syzbot+06fa1063cca8163ea541@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=06fa1063cca8163ea541
Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
---
 drivers/tty/vt/vt.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 156efda7c80d..0d3d602ae147 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -2856,7 +2856,7 @@ static int do_con_write(struct tty_struct *tty, const u8 *buf, int count)
 	struct vt_notifier_param param;
 	bool rescan;
 
-	if (in_interrupt())
+	if (in_interrupt() || irqs_disabled())
 		return count;
 
 	console_lock();
@@ -3314,7 +3314,7 @@ static void con_flush_chars(struct tty_struct *tty)
 {
 	struct vc_data *vc;
 
-	if (in_interrupt())	/* from flush_to_ldisc */
+	if (in_interrupt() || irqs_disabled()) /* from flush_to_ldisc */
 		return;
 
 	/* if we race with con_close(), vt may be null */
-- 
2.18.4