net: do not delay dst_entries_add() in dst_release()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: net: do not delay dst_entries_add() in dst_release() dst_entries_add() uses per-cpu data that might be freed at netns dismantle from ip6_route_net_exit() calling dst_entries_destroy() Before ip6_route_net_exit() can be called, we release all the dsts associated with this netns, via calls to dst_release(), which waits an rcu grace period before calling dst_destroy() dst_entries_add() use in dst_destroy() is racy, because dst_entries_destroy() could have been called already. Decrementing the number of dsts must happen sooner. Notes: 1) in CONFIG_XFRM case, dst_destroy() can call dst_release_immediate(child), this might also cause UAF if the child does not have DST_NOCOUNT set. IPSEC maintainers might take a look and see how to address this. 2) There is also discussion about removing this count of dst, which might happen in future kernels.

References

• https://git.kernel.org/stable/c/547087307bc19417b4f2bc85ba9664a3e8db5a6a
• https://git.kernel.org/stable/c/e3915f028b1f1c37e87542e5aadd33728c259d96
• https://git.kernel.org/stable/c/a60db84f772fc3a906c6c4072f9207579c41166f
• https://git.kernel.org/stable/c/eae7435b48ffc8e9be0ff9cfeae40af479a609dd
• https://git.kernel.org/stable/c/3c7c918ec0aa3555372c5a57f18780b7a96c5cfc
• https://git.kernel.org/stable/c/ac888d58869bb99753e7652be19a151df9ecb35d