[PATCH 4.4 31/53] ttyprintk: fix a potential deadlock in interrupt context issue

Greg Kroah-Hartman
2020-02-03T16:22:46Z

From: Zhenzhong Duan <zhenzhong.duan@gmail.com>

commit 9a655c77ff8fc65699a3f98e237db563b37c439b upstream.

tpk_write()/tpk_close() could be interrupted when holding a mutex, then
in timer handler tpk_write() may be called again trying to acquire same
mutex, lead to deadlock.

Google syzbot reported this issue with CONFIG_DEBUG_ATOMIC_SLEEP
enabled:

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>

See link https://syzkaller.appspot.com/bug?extid=2eeef62ee31f9460ad65 for
more details.

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

Reported-by: syzbot+2eeef62ee31f9460ad65@syzkaller.appspotmail.com
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20200113034842.435-1-zhenzhong.duan@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/char/ttyprintk.c | 15 +++++++++------
1 file changed, 9 insertions(+), 6 deletions(-)

--- a/drivers/char/ttyprintk.c
+++ b/drivers/char/ttyprintk.c
@@ -18,10 +18,11 @@
#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;
@@ -107,11 +108,12 @@ static int tpk_open(struct tty_struct *t
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);
}
@@ -123,13 +125,14 @@ static int tpk_write(struct tty_struct *
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;
}
@@ -179,7 +182,7 @@ 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 |