Merge branch 'ipv6-f6i-fib6_siblings-and-rt-fib6_nsiblings-fixes'

Eric Dumazet says:

====================
ipv6: f6i->fib6_siblings and rt->fib6_nsiblings fixes

Series based on an internal syzbot report with a repro.

After fixing (in the first patch) the original minor issue,
I found that syzbot repro was able to trigger a second
more serious bug in rt6_nlmsg_size().

Code review then led to the two final patches.

I have not released the syzbot bug, because other issues
still need investigations.
====================

Link: https://patch.msgid.link/20250725140725.3626540-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

2 files changed, 57 insertions, 38 deletions

diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index 93578b2ec35f..4d68bd853dba 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -445,15 +445,17 @@ struct fib6_dump_arg {
 static int fib6_rt_dump(struct fib6_info *rt, struct fib6_dump_arg *arg)
 {
 	enum fib_event_type fib_event = FIB_EVENT_ENTRY_REPLACE;
+	unsigned int nsiblings;
 	int err;
 
 	if (!rt || rt == arg->net->ipv6.fib6_null_entry)
 		return 0;
 
-	if (rt->fib6_nsiblings)
+	nsiblings = READ_ONCE(rt->fib6_nsiblings);
+	if (nsiblings)
 		err = call_fib6_multipath_entry_notifier(arg->nb, fib_event,
 							 rt,
-							 rt->fib6_nsiblings,
+							 nsiblings,
 							 arg->extack);
 	else
 		err = call_fib6_entry_notifier(arg->nb, fib_event, rt,
@@ -1138,7 +1140,7 @@ static int fib6_add_rt2node(struct fib6_node *fn, struct fib6_info *rt,
 
 			if (rt6_duplicate_nexthop(iter, rt)) {
 				if (rt->fib6_nsiblings)
-					rt->fib6_nsiblings = 0;
+					WRITE_ONCE(rt->fib6_nsiblings, 0);
 				if (!(iter->fib6_flags & RTF_EXPIRES))
 					return -EEXIST;
 				if (!(rt->fib6_flags & RTF_EXPIRES)) {
@@ -1167,7 +1169,8 @@ static int fib6_add_rt2node(struct fib6_node *fn, struct fib6_info *rt,
 			 */
 			if (rt_can_ecmp &&
 			    rt6_qualify_for_ecmp(iter))
-				rt->fib6_nsiblings++;
+				WRITE_ONCE(rt->fib6_nsiblings,
+					   rt->fib6_nsiblings + 1);
 		}
 
 		if (iter->fib6_metric > rt->fib6_metric)
@@ -1217,7 +1220,8 @@ next_iter:
 		fib6_nsiblings = 0;
 		list_for_each_entry_safe(sibling, temp_sibling,
 					 &rt->fib6_siblings, fib6_siblings) {
-			sibling->fib6_nsiblings++;
+			WRITE_ONCE(sibling->fib6_nsiblings,
+				   sibling->fib6_nsiblings + 1);
 			BUG_ON(sibling->fib6_nsiblings != rt->fib6_nsiblings);
 			fib6_nsiblings++;
 		}
@@ -1264,8 +1268,9 @@ add:
 				list_for_each_entry_safe(sibling, next_sibling,
 							 &rt->fib6_siblings,
 							 fib6_siblings)
-					sibling->fib6_nsiblings--;
-				rt->fib6_nsiblings = 0;
+					WRITE_ONCE(sibling->fib6_nsiblings,
+						   sibling->fib6_nsiblings - 1);
+				WRITE_ONCE(rt->fib6_nsiblings, 0);
 				list_del_rcu(&rt->fib6_siblings);
 				rcu_read_lock();
 				rt6_multipath_rebalance(next_sibling);
@@ -2014,8 +2019,9 @@ static void fib6_del_route(struct fib6_table *table, struct fib6_node *fn,
 			notify_del = true;
 		list_for_each_entry_safe(sibling, next_sibling,
 					 &rt->fib6_siblings, fib6_siblings)
-			sibling->fib6_nsiblings--;
-		rt->fib6_nsiblings = 0;
+			WRITE_ONCE(sibling->fib6_nsiblings,
+				   sibling->fib6_nsiblings - 1);
+		WRITE_ONCE(rt->fib6_nsiblings, 0);
 		list_del_rcu(&rt->fib6_siblings);
 		rt6_multipath_rebalance(next_sibling);
 	}
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 79c8f1acf8a3..aaedc08607c0 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -5346,7 +5346,8 @@ static void ip6_route_mpath_notify(struct fib6_info *rt,
 	 */
 	rcu_read_lock();
 
-	if ((nlflags & NLM_F_APPEND) && rt_last && rt_last->fib6_nsiblings) {
+	if ((nlflags & NLM_F_APPEND) && rt_last &&
+	    READ_ONCE(rt_last->fib6_nsiblings)) {
 		rt = list_first_or_null_rcu(&rt_last->fib6_siblings,
 					    struct fib6_info,
 					    fib6_siblings);
@@ -5670,32 +5671,34 @@ static int rt6_nh_nlmsg_size(struct fib6_nh *nh, void *arg)
 
 static size_t rt6_nlmsg_size(struct fib6_info *f6i)
 {
+	struct fib6_info *sibling;
+	struct fib6_nh *nh;
 	int nexthop_len;
 
 	if (f6i->nh) {
 		nexthop_len = nla_total_size(4); /* RTA_NH_ID */
 		nexthop_for_each_fib6_nh(f6i->nh, rt6_nh_nlmsg_size,
 					 &nexthop_len);
-	} else {
-		struct fib6_nh *nh = f6i->fib6_nh;
-		struct fib6_info *sibling;
-
-		nexthop_len = 0;
-		if (f6i->fib6_nsiblings) {
-			rt6_nh_nlmsg_size(nh, &nexthop_len);
-
-			rcu_read_lock();
+		goto common;
+	}
 
-			list_for_each_entry_rcu(sibling, &f6i->fib6_siblings,
-						fib6_siblings) {
-				rt6_nh_nlmsg_size(sibling->fib6_nh, &nexthop_len);
-			}
+	rcu_read_lock();
+retry:
+	nh = f6i->fib6_nh;
+	nexthop_len = 0;
+	if (READ_ONCE(f6i->fib6_nsiblings)) {
+		rt6_nh_nlmsg_size(nh, &nexthop_len);
 
-			rcu_read_unlock();
+		list_for_each_entry_rcu(sibling, &f6i->fib6_siblings,
+					fib6_siblings) {
+			rt6_nh_nlmsg_size(sibling->fib6_nh, &nexthop_len);
+			if (!READ_ONCE(f6i->fib6_nsiblings))
+				goto retry;
 		}
-		nexthop_len += lwtunnel_get_encap_size(nh->fib_nh_lws);
 	}
-
+	rcu_read_unlock();
+	nexthop_len += lwtunnel_get_encap_size(nh->fib_nh_lws);
+common:
 	return NLMSG_ALIGN(sizeof(struct rtmsg))
 	       + nla_total_size(16) /* RTA_SRC */
 	       + nla_total_size(16) /* RTA_DST */
@@ -5854,7 +5857,7 @@ static int rt6_fill_node(struct net *net, struct sk_buff *skb,
 		if (dst->lwtstate &&
 		    lwtunnel_fill_encap(skb, dst->lwtstate, RTA_ENCAP, RTA_ENCAP_TYPE) < 0)
 			goto nla_put_failure;
-	} else if (rt->fib6_nsiblings) {
+	} else if (READ_ONCE(rt->fib6_nsiblings)) {
 		struct fib6_info *sibling;
 		struct nlattr *mp;
 
@@ -5956,16 +5959,21 @@ static bool fib6_info_uses_dev(const struct fib6_info *f6i,
 	if (f6i->fib6_nh->fib_nh_dev == dev)
 		return true;
 
-	if (f6i->fib6_nsiblings) {
-		struct fib6_info *sibling, *next_sibling;
+	if (READ_ONCE(f6i->fib6_nsiblings)) {
+		const struct fib6_info *sibling;
 
-		list_for_each_entry_safe(sibling, next_sibling,
-					 &f6i->fib6_siblings, fib6_siblings) {
-			if (sibling->fib6_nh->fib_nh_dev == dev)
+		rcu_read_lock();
+		list_for_each_entry_rcu(sibling, &f6i->fib6_siblings,
+					fib6_siblings) {
+			if (sibling->fib6_nh->fib_nh_dev == dev) {
+				rcu_read_unlock();
 				return true;
+			}
+			if (!READ_ONCE(f6i->fib6_nsiblings))
+				break;
 		}
+		rcu_read_unlock();
 	}
-
 	return false;
 }
 
@@ -6321,8 +6329,9 @@ errout:
 void inet6_rt_notify(int event, struct fib6_info *rt, struct nl_info *info,
 		     unsigned int nlm_flags)
 {
-	struct sk_buff *skb;
 	struct net *net = info->nl_net;
+	struct sk_buff *skb;
+	size_t sz;
 	u32 seq;
 	int err;
 
@@ -6330,17 +6339,21 @@ void inet6_rt_notify(int event, struct fib6_info *rt, struct nl_info *info,
 	seq = info->nlh ? info->nlh->nlmsg_seq : 0;
 
 	rcu_read_lock();
-
-	skb = nlmsg_new(rt6_nlmsg_size(rt), GFP_ATOMIC);
+	sz = rt6_nlmsg_size(rt);
+retry:
+	skb = nlmsg_new(sz, GFP_ATOMIC);
 	if (!skb)
 		goto errout;
 
 	err = rt6_fill_node(net, skb, rt, NULL, NULL, NULL, 0,
 			    event, info->portid, seq, nlm_flags);
 	if (err < 0) {
-		/* -EMSGSIZE implies BUG in rt6_nlmsg_size() */
-		WARN_ON(err == -EMSGSIZE);
 		kfree_skb(skb);
+		/* -EMSGSIZE implies needed space grew under us. */
+		if (err == -EMSGSIZE) {
+			sz = max(rt6_nlmsg_size(rt), sz << 1);
+			goto retry;
+		}
 		goto errout;
 	}