[PATCH net-next v3 1/2] ipv6: update NUD_FAILED neighbors from NA messages

[Lawrence Lee <lfqlee314@gmail.com>]

Transition a FAILED neighbor entry to STALE upon receipt of an NA
message on routers when accept_untracked_na is enabled. This extends the
RFC 9131 accept_untracked_na behavior so that FAILED entries are treated
the same as non-existent entries.

RFC 4861 section 7.3.3 says that an entry should be deleted when address
resolution fails. Linux instead retains the entry in NUD_FAILED, so
treating it as untracked is consistent with the protocol model.

Trying to resolve FAILED neighbors via periodic probing (e.g. using
NTF_EXT_MANAGED) is more work compared to this approach which uses
information in NAs that the kernel may already be receiving. Note that
because this behavior in IPv6 is dependent on the accept_untracked_na
sysctl setting, this approach is more conservative than IPv4 which
transitions FAILED neighbors to STALE by default upon receiving GARPs.

Link: https://lore.kernel.org/r/20260813233344.445265-1-lfqlee314@gmail.com
Assisted-by: LLM Sashiko sparse
Signed-off-by: Lawrence Lee <lfqlee314@gmail.com>
---
The existing 6LoWPAN override-only handling also applies to INCOMPLETE
entries and is intentionally left unchanged. The existing
rt6_clean_tohost() source-versus-target behavior applies to all neighbor
states and is also left unchanged.

 Documentation/networking/ip-sysctl.rst | 28 ++++++++-------
 net/ipv6/ndisc.c                       | 47 ++++++++++++++------------
 2 files changed, 41 insertions(+), 34 deletions(-)

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index f7af0286341c..685c84cf543d 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -3225,18 +3225,19 @@ drop_unsolicited_na - BOOLEAN
 	Default: 0 (disabled).
 
 accept_untracked_na - INTEGER
-	Define behavior for accepting neighbor advertisements from devices that
-	are absent in the neighbor cache:
+	Define behavior for accepting neighbor advertisements for IPv6 addresses
+	that are absent from the neighbor cache or whose entries are in FAILED
+	state:
 
-	- 0 - (default) Do not accept unsolicited and untracked neighbor
-	  advertisements.
+	- 0 - (default) Do not create new neighbor cache entries or update
+	  FAILED entries from neighbor advertisements.
 
-	- 1 - Add a new neighbor cache entry in STALE state for routers on
-	  receiving a neighbor advertisement (either solicited or unsolicited)
-	  with target link-layer address option specified if no neighbor entry
-	  is already present for the advertised IPv6 address. Without this knob,
-	  NAs received for untracked addresses (absent in neighbor cache) are
-	  silently ignored.
+	- 1 - For routers, add a new neighbor cache entry or update an existing
+	  FAILED entry to STALE upon receiving a neighbor advertisement (either
+	  solicited or unsolicited) with the target link-layer address option
+	  specified. Without this knob, NAs received for untracked addresses
+	  (absent from the neighbor cache or in FAILED state) are silently
+	  ignored.
 
 	  This is as per router-side behavior documented in RFC9131.
 
@@ -3251,9 +3252,10 @@ accept_untracked_na - INTEGER
 	  used in conjunction with the ndisc_notify setting on the host to
 	  satisfy this prerequisite.
 
-	- 2 - Extend option (1) to add a new neighbor cache entry only if the
-	  source IP address is in the same subnet as an address configured on
-	  the interface that received the neighbor advertisement.
+	- 2 - Extend option (1) to add a new neighbor cache entry or update a
+	  FAILED entry only if the source IP address is in the same subnet as
+	  an address configured on the interface that received the neighbor
+	  advertisement.
 
 enhanced_dad - BOOLEAN
 	Include a nonce option in the IPv6 neighbor solicitation messages used for
diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
index 90cd5d852569..12d85d7f8234 100644
--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c
@@ -973,13 +973,13 @@ static enum skb_drop_reason ndisc_recv_ns(struct sk_buff *skb)
 static int accept_untracked_na(struct inet6_dev *idev, struct in6_addr *saddr)
 {
 	switch (READ_ONCE(idev->cnf.accept_untracked_na)) {
-	case 0: /* Don't accept untracked na (absent in neighbor cache) */
+	case 0: /* Don't accept untracked NA (absent or FAILED) */
 		return 0;
-	case 1: /* Create new entries from na if currently untracked */
+	case 1: /* Create new or update FAILED entries from NA */
 		return 1;
-	case 2: /* Create new entries from untracked na only if saddr is in the
+	case 2: /* Create new or update FAILED entries only if saddr is in the
 		 * same subnet as an address configured on the interface that
-		 * received the na
+		 * received the NA
 		 */
 		return !!ipv6_chk_prefix(saddr, idev->dev);
 	default:
@@ -1067,34 +1067,39 @@ static enum skb_drop_reason ndisc_recv_na(struct sk_buff *skb)
 	neigh = neigh_lookup(tbl, &msg->target, dev);
 
 	/* RFC 9131 updates original Neighbour Discovery RFC 4861.
-	 * NAs with Target LL Address option without a corresponding
-	 * entry in the neighbour cache can now create a STALE neighbour
-	 * cache entry on routers.
+	 * NAs with Target LL Address option can now create a STALE neighbor
+	 * cache entry on routers if the NA does not have a corresponding entry
+	 * in the neighbour cache or has a corresponding FAILED entry.
 	 *
-	 *   entry accept  fwding  solicited        behaviour
-	 * ------- ------  ------  ---------    ----------------------
-	 * present      X       X         0     Set state to STALE
-	 * present      X       X         1     Set state to REACHABLE
-	 *  absent      0       X         X     Do nothing
-	 *  absent      1       0         X     Do nothing
-	 *  absent      1       1         X     Add a new STALE entry
+	 *       entry accept  fwding  solicited        behaviour
+	 * ----------- ------  ------  ---------    ----------------------
+	 *  non-FAILED      X       X         0     Set state to STALE
+	 *  non-FAILED      X       X         1     Set state to REACHABLE
+	 *      FAILED      0       X         X     Do nothing
+	 *      FAILED      1       0         X     Do nothing
+	 *      FAILED      1       1         X     Set state to STALE
+	 *      absent      0       X         X     Do nothing
+	 *      absent      1       0         X     Do nothing
+	 *      absent      1       1         X     Add a new STALE entry
 	 *
 	 * Note that we don't do a (daddr == all-routers-mcast) check.
 	 */
 	new_state = msg->icmph.icmp6_solicited ? NUD_REACHABLE : NUD_STALE;
-	if (!neigh && lladdr && idev && READ_ONCE(idev->cnf.forwarding)) {
-		if (accept_untracked_na(idev, saddr)) {
-			neigh = neigh_create(tbl, &msg->target, dev);
-			new_state = NUD_STALE;
+	if (!neigh || (READ_ONCE(neigh->nud_state) & NUD_FAILED)) {
+		if (!lladdr || !idev || !READ_ONCE(idev->cnf.forwarding) ||
+		    !accept_untracked_na(idev, saddr)) {
+			if (neigh)
+				neigh_release(neigh);
+			return reason;
 		}
+		if (!neigh)
+			neigh = neigh_create(tbl, &msg->target, dev);
+		new_state = NUD_STALE;
 	}
 
 	if (neigh && !IS_ERR(neigh)) {
 		u8 old_flags = neigh->flags;
 
-		if (READ_ONCE(neigh->nud_state) & NUD_FAILED)
-			goto out;
-
 		/*
 		 * Don't update the neighbor cache entry on a proxy NA from
 		 * ourselves because either the proxied node is off link or it
-- 
2.43.0