[PATCH 6.18 60/83] vxlan: keep the last remote linked during FDB flush

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

6.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Kyle Zeng <kylebot@openai.com>

commit d5d4a7b538b52db63927773a8905fcd9f78a42e2 upstream.

A non-nexthop FDB entry is expected to have at least one remote while it
remains reachable through the FDB hash table. A filtered bulk flush
violates this invariant when every remote matches: It unlinks the last
remote in vxlan_fdb_dst_destroy() and only afterwards tells vxlan_flush()
to destroy the parent FDB entry.

An RCU reader can find the parent during this interval.
first_remote_rcu() then applies list_entry_rcu() to the empty list head,
producing an invalid remote pointer that the receive learning path can
read from and write to.

When a matching remote is the sole remaining remote, leave it linked and
ask the caller to destroy the entire FDB entry. vxlan_fdb_destroy() keeps
the remote attached while sending the deletion notification and removing
the parent from the lookup structures.

Fixes: c499fccb71cb ("vxlan: vxlan_core: Support FDB flushing by destination VNI")
Cc: stable@vger.kernel.org
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Co-developed-by: David Lee <david.lee@trailofbits.com>
Signed-off-by: David Lee <david.lee@trailofbits.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260810144115.821654-1-david.lee@trailofbits.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/net/vxlan/vxlan_core.c |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

--- a/drivers/net/vxlan/vxlan_core.c
+++ b/drivers/net/vxlan/vxlan_core.c
@@ -3060,18 +3060,19 @@ vxlan_fdb_flush_match_remotes(struct vxl
 			      const struct vxlan_fdb_flush_desc *desc,
 			      bool *p_destroy_fdb)
 {
-	bool remotes_flushed = false;
 	struct vxlan_rdst *rd, *tmp;
 
 	list_for_each_entry_safe(rd, tmp, &f->remotes, list) {
 		if (!vxlan_fdb_flush_remote_matches(desc, rd))
 			continue;
 
+		if (list_is_singular(&f->remotes)) {
+			*p_destroy_fdb = true;
+			return;
+		}
+
 		vxlan_fdb_dst_destroy(vxlan, f, rd, true);
-		remotes_flushed = true;
 	}
-
-	*p_destroy_fdb = remotes_flushed && list_empty(&f->remotes);
 }
 
 /* Purge the forwarding table */