local_mac_show() loads nt->np.dev and dereferences it with no lock. It can race with the interface going away and netconsole_netdev_event() eventually calling netdev_put() and freeing the device before we dereference it, leading to a UAF. The window is only a few instructions wide, so I could only reproduce it with an artificially widened window between the load and the dereference: BUG: KASAN: slab-use-after-free in local_mac_show+0x4e/0x80 Read of size 8 at addr ffff88800e990458 by task uaf/130 local_mac_show+0x4e/0x80 configfs_read_iter+0x187/0x260 vfs_read+0x453/0x5a0 Allocated by task 116: alloc_netdev_mqs+0x78/0x830 rtnl_create_link+0x53b/0x5c0 Freed by task 121: kfree+0x159/0x420 device_release+0x77/0x120 kobject_put+0xb5/0x160 netdev_run_todo+0x420/0x850 rtnl_dellink+0x259/0x5b0 Two places can free np.dev: (1) netconsole_netdev_event() moves a target out of STATE_ENABLED under target_list_lock before netdev_put(), and (2) process_resume_target() releases np.dev under dynamic_netconsole_mutex. Fixes: 0953864160bd ("[NETPOLL]: no need to store local_mac") Signed-off-by: Gustavo Luiz Duarte --- drivers/net/netconsole.c | 19 ++++++++++++++++--- 1 file changed, 16 insertions(+), 3 deletions(-) diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c index 1fb846ad07d1..be7eb632ee42 100644 --- a/drivers/net/netconsole.c +++ b/drivers/net/netconsole.c @@ -888,10 +888,23 @@ static ssize_t remote_ip_show(struct config_item *item, char *buf) static ssize_t local_mac_show(struct config_item *item, char *buf) { - struct net_device *dev = to_target(item)->np.dev; - static const u8 bcast[ETH_ALEN] = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff }; + struct netconsole_target *nt = to_target(item); + unsigned long flags; + u8 mac[ETH_ALEN]; + + eth_broadcast_addr(mac); + + dynamic_netconsole_mutex_lock(); + /* Hold target_list_lock here so netconsole_netdev_event() doesn't tear + * down np.dev from under us. + */ + spin_lock_irqsave(&target_list_lock, flags); + if (nt->state == STATE_ENABLED && nt->np.dev) + memcpy(mac, nt->np.dev->dev_addr, ETH_ALEN); + spin_unlock_irqrestore(&target_list_lock, flags); + dynamic_netconsole_mutex_unlock(); - return sysfs_emit(buf, "%pM\n", dev ? dev->dev_addr : bcast); + return sysfs_emit(buf, "%pM\n", mac); } static ssize_t remote_mac_show(struct config_item *item, char *buf) -- 2.56.0