[PATCH] tty: vt: make do_con_write() no-op if IRQ is disabled

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

syzbot is reporting that an unprivileged user who logged in from tty
console can crash the system using a reproducer shown below [1], for
commit f9e053dcfc02b0ad ("tty: Serialize tty flow control changes with
flow_lock") changed __start_tty() from schedulable context to atomic
context without realizing that drivers/tty/n_hdlc.c is calling
do_con_write() (which waits for console semaphore) from __start_tty().

----------
  #include <sys/ioctl.h>
  #include <unistd.h>

  int main(int argc, char *argv[])
  {
    const int disc = 0xd;

    ioctl(1, TIOCSETD, &disc);
    while (1) {
      ioctl(1, TCXONC, 0);
      write(1, "", 1);
      ioctl(1, TCXONC, 1); /* Kernel panic - not syncing: scheduling while atomic */
    }
  }
----------

While we would need to bring __start_tty() back to schedulable context
in order to be able to call do_con_write() from ioctl(TCXONC, TCOON),
changes required might be complicated; an oversight will result in a
new sleep from atomic context bug or a new race window bug. Therefore,
firstly let's fix crash problem by protecting do_con_write() side.

Link: https://syzkaller.appspot.com/bug?extid=5f47a8cea6a12b77a876 [1]
Reported-by: syzbot <syzbot+5f47a8cea6a12b77a876@syzkaller.appspotmail.com>
Analyzed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Fixes: f9e053dcfc02b0ad ("tty: Serialize tty flow control changes with flow_lock")
---
 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 7359c3e80d63..efa16f51c978 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -2902,7 +2902,7 @@ static int do_con_write(struct tty_struct *tty, const unsigned char *buf, int co
 	struct vt_notifier_param param;
 	bool rescan;
 
-	if (in_interrupt())
+	if (in_interrupt() || irqs_disabled())
 		return count;
 
 	console_lock();
@@ -3358,7 +3358,7 @@ 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 */
-- 
2.32.0