[PATCH net] net: ipv6: fix NOREF dst use in seg6 and rpl lwtunnels

Andrea Mayer
2026-04-21T09:49:20Z

seg6_input_core() and rpl_input() call ip6_route_input() which sets a
NOREF dst on the skb, then pass it to dst_cache_set_ip6() invoking
dst_hold() unconditionally.
On PREEMPT_RT, ksoftirqd is preemptible and a higher-priority task can
release the underlying pcpu_rt between the lookup and the caching
through a concurrent FIB lookup on a shared nexthop.
Simplified race sequence:

ksoftirqd/X higher-prio task (same CPU X)
----------- --------------------------------
seg6_input_core(,skb)/rpl_input(skb)
dst_cache_get()
-> miss
ip6_route_input(skb)
-> ip6_pol_route(,skb,flags)
[RT6_LOOKUP_F_DST_NOREF in flags]
-> FIB lookup resolves fib6_nh
[nhid=N route]
-> rt6_make_pcpu_route()
[creates pcpu_rt, refcount=1]
pcpu_rt->sernum = fib6_sernum
[fib6_sernum=W]
-> cmpxchg(fib6_nh.rt6i_pcpu,
NULL, pcpu_rt)
[slot was empty, store succeeds]
-> skb_dst_set_noref(skb, dst)
[dst is pcpu_rt, refcount still 1]

rt_genid_bump_ipv6()
-> bumps fib6_sernum
[fib6_sernum from W to Z]
ip6_route_output()
-> ip6_pol_route()
-> FIB lookup resolves fib6_nh
[nhid=N]
-> rt6_get_pcpu_route()
pcpu_rt->sernum != fib6_sernum
[W <> Z, stale]
-> prev = xchg(rt6i_pcpu, NULL)
-> dst_release(prev)
[prev is pcpu_rt,
refcount 1->0, dead]

dst = skb_dst(skb)
[dst is the dead pcpu_rt]
dst_cache_set_ip6(dst)
-> dst_hold() on dead dst
-> WARN / use-after-free

For the race to occur, ksoftirqd must be preemptible (PREEMPT_RT without
PREEMPT_RT_NEEDS_BH_LOCK) and a concurrent task must be able to release
the pcpu_rt. Shared nexthop objects provide such a path, as two routes
pointing to the same nhid share the same fib6_nh and its rt6i_pcpu
entry.

Fix seg6_input_core() and rpl_input() by calling skb_dst_force() after
ip6_route_input() to force the NOREF dst into a refcounted one before
caching.
The output path is not affected as ip6_route_output() already returns a
refcounted dst.

Fixes: af4a2209b134 ("ipv6: sr: use dst_cache in seg6_input")
Fixes: a7a29f9c361f ("net: ipv6: add rpl sr tunnel")
Cc: stable@vger.kernel.org
Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
---
net/ipv6/rpl_iptunnel.c | 9 +++++++++
net/ipv6/seg6_iptunnel.c | 9 +++++++++
2 files changed, 18 insertions(+)

diff --git a/net/ipv6/rpl_iptunnel.c b/net/ipv6/rpl_iptunnel.c
index c7942cf65567..4e10adcd70e8 100644
--- a/net/ipv6/rpl_iptunnel.c
+++ b/net/ipv6/rpl_iptunnel.c
@@ -287,7 +287,16 @@ static int rpl_input(struct sk_buff *skb)

if (!dst) {
ip6_route_input(skb);
+
+ /* ip6_route_input() sets a NOREF dst; force a refcount on it
+ * before caching or further use.
+ */
+ skb_dst_force(skb);
dst = skb_dst(skb);
+ if (unlikely(!dst)) {
+ err = -ENETUNREACH;
+ goto drop;
+ }

/* cache only if we don't create a dst reference loop */
if (!dst->error && lwtst != dst->lwtstate) {
diff --git a/net/ipv6/seg6_iptunnel.c b/net/ipv6/seg6_iptunnel.c
index 97b50d9b1365..94284b483be0 100644
--- a/net/ipv6/seg6_iptunnel.c
+++ b/net/ipv6/seg6_iptunnel.c
@@ -515,7 +515,16 @@ static int seg6_input_core(struct net *net, struct sock *sk,

if (!dst) {
ip6_route_input(skb);
+
+ /* ip6_route_input() sets a NOREF dst; force a refcount on it
+ * before caching or further use.
+ */
+ skb_dst_force(skb);
dst = skb_dst(skb);
+ if (unlikely(!dst)) {
+ err = -ENETUNREACH;
+ goto drop;
+ }

/* cache only if we don't create a dst reference loop */
if (!dst->error && lwtst != dst->lwtstate) {
--
2.20.1