[PATCH 5.15 078/570] ipv6: fix NULL pointer deref in ip6_rt_get_dev_rcu()

Greg Kroah-Hartman
2026-04-13T16:21:28Z

From: Jakub Kicinski <kuba@kernel.org>

[ Upstream commit 2ffb4f5c2ccb2fa1c049dd11899aee7967deef5a ]

l3mdev_master_dev_rcu() can return NULL when the slave device is being
un-slaved from a VRF. All other callers deal with this, but we lost
the fallback to loopback in ip6_rt_pcpu_alloc() -> ip6_rt_get_dev_rcu()
with commit 4832c30d5458 ("net: ipv6: put host and anycast routes on
device with address").

KASAN: null-ptr-deref in range [0x0000000000000108-0x000000000000010f]
RIP: 0010:ip6_rt_pcpu_alloc (net/ipv6/route.c:1418)
Call Trace:
ip6_pol_route (net/ipv6/route.c:2318)
fib6_rule_lookup (net/ipv6/fib6_rules.c:115)
ip6_route_output_flags (net/ipv6/route.c:2607)
vrf_process_v6_outbound (drivers/net/vrf.c:437)

I was tempted to rework the un-slaving code to clear the flag first
and insert synchronize_rcu() before we remove the upper. But looks like
the explicit fallback to loopback_dev is an established pattern.
And I guess avoiding the synchronize_rcu() is nice, too.

Fixes: 4832c30d5458 ("net: ipv6: put host and anycast routes on device with address")
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20260301194548.927324-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/ipv6/route.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index f30a5b7d93f4d..05e2ea8b269df 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1018,7 +1018,8 @@ static struct net_device *ip6_rt_get_dev_rcu(const struct fib6_result *res)
*/
if (netif_is_l3_slave(dev) &&
!rt6_need_strict(&res->f6i->fib6_dst.addr))
- dev = l3mdev_master_dev_rcu(dev);
+ dev = l3mdev_master_dev_rcu(dev) ? :
+ dev_net(dev)->loopback_dev;
else if (!netif_is_l3_master(dev))
dev = dev_net(dev)->loopback_dev;
/* last case is netif_is_l3_master(dev) is true in which
--
2.51.0