CVE-2026-53012

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

nexthop: fix IPv6 route referencing IPv4 nexthop

syzbot reported a panic [1] [2].

When an IPv6 nexthop is replaced with an IPv4 nexthop, the hasv4 flag of all groups containing this nexthop is not updated. This is because nhgroupv4update is only called when replacing AFINET to AFINET6, but the reverse direction (AFINET6 to AFINET) is missed.

This allows a stale hasv4=false to bypass fib6checknexthop, causing IPv6 routes to be attached to groups that effectively contain only AFINET members. Subsequent route lookups then call nexthopfib6nh() which returns NULL for the AF_INET member, leading to a NULL pointer dereference.

Fix by calling nhgroupv4update whenever the family changes, not just AFINET to AF_INET6.

Reproducer: # AFINET6 blackhole ip -6 nexthop add id 1 blackhole # group with hasv4=false ip nexthop add id 100 group 1 # replace with AFINET (no -6), hasv4 stays false ip nexthop replace id 1 blackhole # pass stale has_v4 check ip -6 route add 2001:db8::/64 nhid 100 # panic ping -6 2001:db8::1

[1] https://syzkaller.appspot.com/bug?id=e17283eb2f8dcf3dd9b47fe6f67a95f71faadad0 [2] https://syzkaller.appspot.com/bug?id=8699b6ae54c9f35837d925686208402949e12ef3