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, l2tp_session_free drops the tunnel

refcount if session->tunnel is non-NULL. However, session->tunnel is

set in l2tp_session_create, before the tunnel refcount is incremented

by l2tp_session_register, which leaves a small window where

session->tunnel is non-NULL when the tunnel refcount hasn't been

bumped.

Moving the assignment to l2tp_session_register is trivial but

l2tp_session_create calls l2tp_session_set_header_len which uses

session->tunnel to get the tunnel's encap. Add an encap arg to

l2tp_session_set_header_len to avoid using session->tunnel.

If l2tpv3 sessions have colliding IDs, it is possible for

l2tp_v3_session_get to race with l2tp_session_register and fetch a

session which doesn't yet have session->tunnel set. Add a check for

this case.