[PATCH net] ip6mr: do not clone dst in ip6mr_cache_report()

[Eric Dumazet <edumazet@google.com>]

IPv6 input attaches a non-refcounted (NOREF) dst to skbs under RCU.
When an ingress multicast packet misses MFC lookup,
ip6mr_cache_unresolved() places the skb onto the unresolved queue,
escaping the receive-side RCU grace period.

If the underlying route is deleted and freed, and the MFC queue is later
resolved with a wrong parent interface, ip6_mr_forward() invokes
ip6mr_cache_report(..., MRT6MSG_WRONGMIF), which executes
dst_clone(skb_dst(pkt)) on the freed dst entry, triggering a slab
use-after-free.

Report packets queued to mroute6_sk (a raw socket) and netlink
notifications do not require an attached dst entry.

Fix this by:
1. Removing dst_clone() in ip6mr_cache_report() and ensuring report skbs
   do not hold a dst.
2. Dropping skb_dst before queuing unresolved skbs in
   ip6mr_cache_unresolved(), matching the fact that multicast
   forwarding resolves outgoing routes anew via ip6_route_output().

Fixes: 67f415dd2906 ("ipv6: convert rx data path to not take refcnt on dst")
Reported-by: Zero Day Initiative <zdi-disclosures@trendmicro.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/ipv6/ip6mr.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
index 604a58838901a74712d08505c6bbbdeafd28149c..3f2ed9b77deb51799f34e3826ae271d8d3e2a2dd 100644
--- a/net/ipv6/ip6mr.c
+++ b/net/ipv6/ip6mr.c
@@ -1162,10 +1162,10 @@ static int ip6mr_cache_report(const struct mr_table *mrt, struct sk_buff *pkt,
 	msg->im6_src = ipv6_hdr(pkt)->saddr;
 	msg->im6_dst = ipv6_hdr(pkt)->daddr;
 
-	skb_dst_set(skb, dst_clone(skb_dst(pkt)));
 	skb->ip_summed = CHECKSUM_UNNECESSARY;
 	}
 
+	skb_dst_drop(skb);
 	mrt6msg_netlink_event(mrt, skb);
 
 	/* Deliver to user space multicast routing algorithms */
@@ -1246,6 +1246,7 @@ static int ip6mr_cache_unresolved(struct mr_table *mrt, mifi_t mifi,
 		skb->skb_iif = dev->ifindex;
 	}
 
+	skb_dst_drop(skb);
 	skb_queue_tail(&c->_c.mfc_un.unres.unresolved, skb);
 
 	spin_unlock_bh(&mfc_unres_lock);
-- 
2.55.0.737.g08866a6d13-goog