CVE-2024-49940

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

l2tp: prevent possible tunnel refcount underflow

When a session is created, it sets a backpointer to its tunnel. When the session refcount drops to 0, l2tpsessionfree drops the tunnel refcount if session->tunnel is non-NULL. However, session->tunnel is set in l2tpsessioncreate, before the tunnel refcount is incremented by l2tpsessionregister, which leaves a small window where session->tunnel is non-NULL when the tunnel refcount hasn't been bumped.

Moving the assignment to l2tpsessionregister is trivial but l2tpsessioncreate calls l2tpsessionsetheaderlen which uses session->tunnel to get the tunnel's encap. Add an encap arg to l2tpsessionsetheaderlen to avoid using session->tunnel.

If l2tpv3 sessions have colliding IDs, it is possible for l2tpv3sessionget to race with l2tpsession_register and fetch a session which doesn't yet have session->tunnel set. Add a check for this case.