[PATCH net] ppp: ensure file->private_data can't be overridden

Lock ppp_mutex and check that file->private_data is NULL before
executing any action in ppp_unattached_ioctl().
The test done by ppp_ioctl() can't be relied upon, because
file->private_data may have been updated meanwhile. In which case
ppp_unattached_ioctl() will override file->private_data and mess up
reference counters or loose pointer to previously allocated PPP unit.

In case the test fails, -ENOTTY is returned, just like if ppp_ioctl()
had rejected the ioctl in the first place.

Signed-off-by: Guillaume Nault <redacted>
---
This seems to originate in BKL removal :
f3ff8a4d80e8 ("ppp: push BKL down into the driver") moved the test on
file->private_data (in ppp_ioctl()) out of BKL protection. BKL was then
replaced by ppp_mutex in 15fd0cd9a2ad ("net: autoconvert trivial BKL
users to private mutex").

 drivers/net/ppp/ppp_generic.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index d61da9ec..d1dbcb6 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c

@@ -845,6 +845,11 @@ static int ppp_unattached_ioctl(struct net *net, struct ppp_file *pf,
 	int __user *p = (int __user *)arg;
 
 	mutex_lock(&ppp_mutex);
+	if (file->private_data) {
+		mutex_unlock(&ppp_mutex);
+		return -ENOTTY;
+	}
+
 	switch (cmd) {
 	case PPPIOCNEWUNIT:
 		/* Create a new ppp unit */

-- 
2.7.0