[PATCH net-next 2/4] netconsole: read np.dev under the RTNL in local_mac_show()

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

Fixes: 0953864160bd ("[NETPOLL]: no need to store local_mac")
Signed-off-by: Gustavo Luiz Duarte <redacted>
---
 drivers/net/netconsole.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 7dde00eb8e98..5f4311726d11 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c

@@ -900,10 +900,17 @@ 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);
+	int ret;
 
-	return sysfs_emit(buf, "%pM\n", dev ? dev->dev_addr : bcast);
+	/* Hold RTNL here so netconsole_netdev_event() doesn't tear down np.dev
+	 * from under us.
+	 */
+	rtnl_lock();
+	ret = sysfs_emit(buf, "%pM\n", nt->np.dev ? nt->np.dev->dev_addr : bcast);
+	rtnl_unlock();
+	return ret;
 }
 
 static ssize_t remote_mac_show(struct config_item *item, char *buf)

-- 
2.55.0