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 <[email protected]> > --- > net/ipv4/nexthop.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) >
Reviewed-by: David Ahern <[email protected]>
