CVE-2024-49940 Information

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.

Reference

https://git.kernel.org/stable/c/f7415e60c25a6108cd7955a20b2e66b6251ffe02 https://git.kernel.org/stable/c/24256415d18695b46da06c93135f5b51c548b950