CVE-2024-49940l2tp: prevent possible tunnel refcount underflow

Official description

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/aa7132362e79abc4f2add4d8657dd1679341edc5
• https://git.kernel.org/stable/c/98691b2e28a6c3dac0680914493d54c77f215755
• https://git.kernel.org/stable/c/f7415e60c25a6108cd7955a20b2e66b6251ffe02
• https://git.kernel.org/stable/c/24256415d18695b46da06c93135f5b51c548b950