On 2021/04/12 19:44, Greg Kroah-Hartman wrote:
> And trying to "open exclusive only" just does not work, the kernel can
> not enforce that at all, sorry. Any driver that you see trying to do
> that is trivial to work around in userspace, making the kernel code
> pointless.
You mean something like below cannot be used?
diff --git a/drivers/char/ttyprintk.c b/drivers/char/ttyprintk.c
index 6a0059e508e3..57200569918a 100644
--- a/drivers/char/ttyprintk.c
+++ b/drivers/char/ttyprintk.c
@@ -84,14 +84,26 @@ static int tpk_printk(const unsigned char *buf, int count)
return count;
}
+static DEFINE_MUTEX(open_close_lock);
+
/*
* TTY operations open function.
*/
static int tpk_open(struct tty_struct *tty, struct file *filp)
{
- tty->driver_data = &tpk_port;
+ int ret;
+
+ if (mutex_lock_killable(&open_close_lock))
+ return -EINTR;
- return tty_port_open(&tpk_port.port, tty, filp);
+ if (tpk_port.port.count) {
+ ret = -EBUSY;
+ } else {
+ tty->driver_data = &tpk_port;
+ ret = tty_port_open(&tpk_port.port, tty, filp);
+ }
+ mutex_unlock(&open_close_lock);
+ return ret;
}
/*
@@ -102,12 +114,14 @@ static void tpk_close(struct tty_struct *tty, struct file
*filp)
struct ttyprintk_port *tpkp = tty->driver_data;
unsigned long flags;
+ mutex_lock(&open_close_lock);
spin_lock_irqsave(&tpkp->spinlock, flags);
/* flush tpk_printk buffer */
tpk_printk(NULL, 0);
spin_unlock_irqrestore(&tpkp->spinlock, flags);
tty_port_close(&tpkp->port, tty, filp);
+ mutex_unlock(&open_close_lock);
}
/*
> Like any tty port, if you have multiple accesses, all bets are off and
> hilarity ensues. Just don't do that and expect things to be working
> well.
Since syzkaller is a fuzzer, syzkaller happily opens /dev/ttyprintk from
multiple threads. Should we update syzkaller to use CONFIG_TTY_PRINTK=n ?
