CVE-2026-98306: seg6: set IPSKB_L3SLAVE from IP6SKB_L3SLAVE on IPIP decapsulation

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

seg6: set IPSKBL3SLAVE from IP6SKBL3SLAVE on IPIP decapsulation

When an SRv6 packet arrives on an interface enslaved to a VRF, vrfip6rcv() sets IP6SKBL3SLAVE in IP6CB, but decapandvalidate() has never set IPSKBL3SLAVE in IPCB. The bit stayed clear in the common case, and with CONFIGIPV6MIP6 the leftover fragmaxsize of a reassembled outer packet could even set it, with no VRF involved. Commit 44930446dde4 ("ipv6: seg6: clear IPv4 control block on IPIP decapsulation") then made the unreliable bit reliably clear.

The effect of the missing flag is visible with End.DX4 when a delivery to a local address of the node reaches the socket lookup. For example, a UDP socket bound to the enslaved ingress interface does not receive any of the decapsulated packets, while an unbound socket outside the VRF does. This contradicts Documentation/networking/vrf.rst: by default the scope of an unbound UDP or TCP socket is limited to the default VRF.

Set IPSKBL3SLAVE for IPv4 in decapandvalidate(), which already does the same for IPv6. The socket lookup then matches the decapsulated packet like any other packet received on that enslaved interface. Such a packet matches an unbound UDP or TCP socket only when udpl3mdevaccept or tcpl3mdevaccept is set.