net/l2tp: fix warning in l2tp_exit_net found by syzbot

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: net/l2tp: fix warning in l2tp_exit_net found by syzbot In l2tp's net exit handler, we check that an IDR is empty before destroying it: WARN_ON_ONCE(!idr_is_empty(&pn->l2tp_tunnel_idr)); idr_destroy(&pn->l2tp_tunnel_idr); By forcing memory allocation failures in idr_alloc_32, syzbot is able to provoke a condition where idr_is_empty returns false despite there being no items in the IDR. This turns out to be because the radix tree of the IDR contains only internal radix-tree nodes and it is this that causes idr_is_empty to return false. The internal nodes are cleaned by idr_destroy. Use idr_for_each to check that the IDR is empty instead of idr_is_empty to avoid the problem.

References

• https://git.kernel.org/stable/c/a487cc8986d6dd75b60b59004f3bd2ea9b4dd541
• https://git.kernel.org/stable/c/5d066766c5f1252f98ff859265bcd1a5b52ac46c