[PATCH net v3] vxlan: keep the last remote linked during FDB flush

[David Lee]

From: Kyle Zeng <kylebot@xxxxxxxxxx>

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@xxxxxxxxxxxxxxx
Suggested-by: Ido Schimmel <idosch@xxxxxxxxxx>
Assisted-by: Codex:gpt-5.6-sol Codex:gpt-5.5-cyber
Signed-off-by: Kyle Zeng <kylebot@xxxxxxxxxx>
Co-developed-by: David Lee <david.lee@xxxxxxxxxxxxxxx>
Signed-off-by: David Lee <david.lee@xxxxxxxxxxxxxxx>
---
Changes in v3:
- Remove remotes_flushed and the redundant final p_destroy_fdb assignment,
  as suggested by Ido Schimmel; the caller initializes the flag to false.

Changes in v2:
- Add the net tree prefix to the subject.
- Restore Kyle Zeng as the patch author and correct the sign-off chain.
- Move the research credit below the commit-message separator.
- Add the recipients reported by netdev CI.

v1: https://lore.kernel.org/all/20260731141311.570187-1-david.lee@xxxxxxxxxxxxxxx/

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

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 | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c
index d834a4865..0c42cfd96 100644
--- a/drivers/net/vxlan/vxlan_core.c
+++ b/drivers/net/vxlan/vxlan_core.c
@@ -3051,18 +3051,19 @@ vxlan_fdb_flush_match_remotes(struct vxlan_fdb *f, struct vxlan_dev *vxlan,
 			      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 */
-- 
2.53.0