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 has_v4 flag

of all groups containing this nexthop is not updated. This is because

nh_group_v4_update is only called when replacing AF_INET to AF_INET6,

but the reverse direction (AF_INET6 to AF_INET) is missed.

This allows a stale has_v4=false to bypass fib6_check_nexthop, causing

IPv6 routes to be attached to groups that effectively contain only AF_INET

members. Subsequent route lookups then call nexthop_fib6_nh() which

returns NULL for the AF_INET member, leading to a NULL pointer

dereference.

Fix by calling nh_group_v4_update whenever the family changes, not just

AF_INET to AF_INET6.

Reproducer:

# AF_INET6 blackhole

ip -6 nexthop add id 1 blackhole

# group with has_v4=false

ip nexthop add id 100 group 1

# replace with AF_INET (no -6), has_v4 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