[PATCH] net: bonding: fix NULL pointer dereference in bond_do_ioctl()

[ZhaoJinming]

In bond_do_ioctl(), slave_dev is obtained via __dev_get_by_name() which
can return NULL if the requested interface name does not exist. However,
the subsequent slave_dbg() call is placed before the NULL check:

    slave_dev = __dev_get_by_name(net, ifr->ifr_slave);
    slave_dbg(bond_dev, slave_dev, "slave_dev=%p:\n", slave_dev); //here
    if (!slave_dev)
        return -ENODEV;

The slave_dbg() macro expands to netdev_dbg(bond_dev, "(slave %s): " fmt,
(slave_dev)->name, ...) which unconditionally dereferences slave_dev->name
before the NULL check is performed. This results in a NULL pointer
dereference kernel oops when a user calls bonding ioctl (e.g. 
SIOCBONDENSLAVE, SIOCBONDRELEASE, etc.) with a non-existent slave 
interface name.

This is reachable from userspace via the bonding ioctl interface with
CAP_NET_ADMIN capability, making it a potential local denial-of-service
vector.

Fix by moving the slave_dbg() call after the NULL check.

Fixes: e2a7420df2e0 ("bonding/main: convert to using slave printk macros")
Cc: stable@xxxxxxxxxxxxxxx # v5.2+
Signed-off-by: ZhaoJinming <zhaojinming@xxxxxxxxxxxxx>
---
 drivers/net/bonding/bond_main.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 82e779f7916b..8e75453ce0ef 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -4621,11 +4621,11 @@ static int bond_do_ioctl(struct net_device *bond_dev, struct ifreq *ifr, int cmd
 
 	slave_dev = __dev_get_by_name(net, ifr->ifr_slave);
 
-	slave_dbg(bond_dev, slave_dev, "slave_dev=%p:\n", slave_dev);
-
 	if (!slave_dev)
 		return -ENODEV;
 
+	slave_dbg(bond_dev, slave_dev, "slave_dev=%p:\n", slave_dev);
+
 	switch (cmd) {
 	case SIOCBONDENSLAVE:
 		res = bond_enslave(bond_dev, slave_dev, NULL);
-- 
2.20.1