[PATCH net] net: read dev->needs_free_netdev before potentially freeing dev

["Jason A. Donenfeld" <Jason@zx2c4.com>]

If dev->needs_free_netdev is true, it means that netdev_run_todo should
call free_netdev(dev) after it calls dev->priv_destructor. If
dev->needs_free_netdev is false, then it means that either
dev->priv_destructor is taking care of calling free_netdev(dev), or
something else, elsewhere, is doing that. In this case, branching on
"if (dev->needs_free_netdev)" after calling dev->priv_destructor is a
potential UaF. This patch fixes the issue by reading
dev->needs_free_netdev before calling dev->priv_destructor.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: cf124db566e6 ("net: Fix inconsistent teardown and release of private netdev state.")
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
I believe that the bug Dan reported would easily be fixed as well by
just setting dev->needs_free_netdev=true and removing the call to
free_netdev(dev) in wg_destruct, in wireguard. If you think that this is
the more proper fix -- and that the problem actually isn't this flow in
dev.c and any code that might hit this UaF is wrong -- let me know and
I'll send in a patch for wireguard instead.

 net/core/dev.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 7df6c9617321..abe53c2fae8c 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10073,6 +10073,8 @@ void netdev_run_todo(void)
 	while (!list_empty(&list)) {
 		struct net_device *dev
 			= list_first_entry(&list, struct net_device, todo_list);
+		bool needs_free_netdev = dev->needs_free_netdev;
+
 		list_del(&dev->todo_list);
 
 		if (unlikely(dev->reg_state != NETREG_UNREGISTERING)) {
@@ -10097,7 +10099,7 @@ void netdev_run_todo(void)
 #endif
 		if (dev->priv_destructor)
 			dev->priv_destructor(dev);
-		if (dev->needs_free_netdev)
+		if (needs_free_netdev)
 			free_netdev(dev);
 
 		/* Report a network device has been unregistered */
-- 
2.28.0