CVE-2024-53211

In the Linux kernel, the following vulnerability has been resolved:

net/l2tp: fix warning in l2tpexitnet 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 idralloc32, syzbot is able to provoke a condition where idrisempty 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 idrisempty to return false. The internal nodes are cleaned by idr_destroy.

Use idrforeach to check that the IDR is empty instead of idrisempty to avoid the problem.