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

https://git.kernel.org/stable/c/29c95185ba32b621fbc3800fb86e7dc3edf5c2be

https://git.kernel.org/stable/c/613c8f4a501421dd258b07ea614205d4e16ec845

https://git.kernel.org/stable/c/6275796f22bb382f3e9aa58ed0b4ef7bdad78cb8

https://git.kernel.org/stable/c/9c2d6770a5f4545a307eb66979bef7656a34d621

https://git.kernel.org/stable/c/aaac3bed034239e1d75732211d9b05f30b0b4f35

https://git.kernel.org/stable/c/ad85961004fd4bd2f31209ac4b07612c6cefb9e7

https://git.kernel.org/stable/c/b3b7e850e1541f0520c4a12ec884255c30427ff6

https://git.kernel.org/stable/c/ceffe81a0be92afc0cd1340bc8ca46559cce9bb4

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