[PATCH] ttyprintk: fix a potential sleeping in interrupt context issue

[Zhenzhong Duan]

Google syzbot reports:
BUG: sleeping function called from invalid context at
kernel/locking/mutex.c:938
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 0, name: swapper/1
1 lock held by swapper/1/0:
...
Call Trace:
  <IRQ>
  dump_stack+0x197/0x210
  ___might_sleep.cold+0x1fb/0x23e
  __might_sleep+0x95/0x190
  __mutex_lock+0xc5/0x13c0
  mutex_lock_nested+0x16/0x20
  tpk_write+0x5d/0x340
  resync_tnc+0x1b6/0x320
  call_timer_fn+0x1ac/0x780
  run_timer_softirq+0x6c3/0x1790
  __do_softirq+0x262/0x98c
  irq_exit+0x19b/0x1e0
  smp_apic_timer_interrupt+0x1a3/0x610
  apic_timer_interrupt+0xf/0x20
  </IRQ>

This patch fix it by using spinlock in process context instead of
mutex and having interrupt disabled in critical section.

Reported-by: syzbot+2eeef62ee31f9460ad65@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>
---
 drivers/char/ttyprintk.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/drivers/char/ttyprintk.c b/drivers/char/ttyprintk.c
index 4f24e46ebe7c..56db949a7b70 100644
--- a/drivers/char/ttyprintk.c
+++ b/drivers/char/ttyprintk.c
@@ -15,10 +15,11 @@
 #include <linux/serial.h>
 #include <linux/tty.h>
 #include <linux/module.h>
+#include <linux/spinlock.h>

 struct ttyprintk_port {
  struct tty_port port;
- struct mutex port_write_mutex;
+ spinlock_t spinlock;
 };

 static struct ttyprintk_port tpk_port;
@@ -99,11 +100,12 @@ static int tpk_open(struct tty_struct *tty,
struct file *filp)
 static void tpk_close(struct tty_struct *tty, struct file *filp)
 {
  struct ttyprintk_port *tpkp = tty->driver_data;
+ unsigned long flags;

- mutex_lock(&tpkp->port_write_mutex);
+ spin_lock_irqsave(&tpkp->spinlock, flags);
  /* flush tpk_printk buffer */
  tpk_printk(NULL, 0);
- mutex_unlock(&tpkp->port_write_mutex);
+ spin_unlock_irqrestore(&tpkp->spinlock, flags);

  tty_port_close(&tpkp->port, tty, filp);
 }
@@ -115,13 +117,14 @@ static int tpk_write(struct tty_struct *tty,
  const unsigned char *buf, int count)
 {
  struct ttyprintk_port *tpkp = tty->driver_data;
+ unsigned long flags;
  int ret;


  /* exclusive use of tpk_printk within this tty */
- mutex_lock(&tpkp->port_write_mutex);
+ spin_lock_irqsave(&tpkp->spinlock, flags);
  ret = tpk_printk(buf, count);
- mutex_unlock(&tpkp->port_write_mutex);
+ spin_unlock_irqrestore(&tpkp->spinlock, flags);

  return ret;
 }
@@ -171,7 +174,7 @@ static int __init ttyprintk_init(void)
 {
  int ret = -ENOMEM;

- mutex_init(&tpk_port.port_write_mutex);
+ spin_lock_init(&tpk_port.spinlock);

  ttyprintk_driver = tty_alloc_driver(1,
  TTY_DRIVER_RESET_TERMIOS |
--
2.23.0