[PATCH] vxlan: keep the last remote linked during FDB flush

[David Lee <david.lee@trailofbits.com>]

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
Bug found and triaged by OpenAI Security Research and 
validated by Trail of Bits.

Assisted-by: Codex:gpt-5.6-sol gpt-5.5-cyber
Signed-off-by: Kyle Zeng <kylebot@openai.com>
---
Trail of Bits has a reproducer for this bug that triggers a
KASAN slab-out-of-bounds read in vxlan_snoop() and can share if needed.

 drivers/net/vxlan/vxlan_core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c
index d834a4865..a00df127d 100644
--- a/drivers/net/vxlan/vxlan_core.c
+++ b/drivers/net/vxlan/vxlan_core.c
@@ -3058,6 +3058,11 @@ vxlan_fdb_flush_match_remotes(struct vxlan_fdb *f, struct vxlan_dev *vxlan,
 		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;
 	}
-- 
2.53.0