Re: [PATCH net v1 1/2] nexthop: fix IPv6 route referencing IPv4 nexthop

David Ahern
2026-04-13T14:46:40Z

On 4/13/26 5:45 AM, Jiayuan Chen wrote:
> 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
> Fixes: 7bf4796dd099 ("nexthops: add support for replace")
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
> net/ipv4/nexthop.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>

Reviewed-by: David Ahern <dsahern@kernel.org>