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

[Andrea Mayer <andrea.mayer@uniroma2.it>]

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