[PATCH 6.1.y] net: bridge: use a stable FDB dst snapshot in RCU readers

Sasha Levin
2026-05-04T10:45:45Z

From: Zhengchuan Liang <zcliangcn@gmail.com>

[ Upstream commit df4601653201de21b487c3e7fffd464790cab808 ]

Local FDB entries can be rewritten in place by `fdb_delete_local()`, which
updates `f->dst` to another port or to `NULL` while keeping the entry
alive. Several bridge RCU readers inspect `f->dst`, including
`br_fdb_fillbuf()` through the `brforward_read()` sysfs path.

These readers currently load `f->dst` multiple times and can therefore
observe inconsistent values across the check and later dereference.
In `br_fdb_fillbuf()`, this means a concurrent local-FDB update can change
`f->dst` after the NULL check and before the `port_no` dereference,
leading to a NULL-ptr-deref.

Fix this by taking a single `READ_ONCE()` snapshot of `f->dst` in each
affected RCU reader and using that snapshot for the rest of the access
sequence. Also publish the in-place `f->dst` updates in `fdb_delete_local()`
with `WRITE_ONCE()` so the readers and writer use matching access patterns.

Fixes: 960b589f86c7 ("bridge: Properly check if local fdb entry can be deleted in br_fdb_change_mac_address")
Cc: stable@kernel.org
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Co-developed-by: Yuan Tan <yuantan098@gmail.com>
Signed-off-by: Yuan Tan <yuantan098@gmail.com>
Suggested-by: Xin Liu <bird@lzu.edu.cn>
Tested-by: Ren Wei <enjou1224z@gmail.com>
Signed-off-by: Zhengchuan Liang <zcliangcn@gmail.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/6570fabb85ecadb8baaf019efe856f407711c7b9.1776043229.git.zcliangcn@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
[ kept combined `BR_PROXYARP_WIFI | BR_NEIGH_SUPPRESS` check and `cb->args[2]` instead of `br_is_neigh_suppress_enabled()` helper and `ctx->fdb_idx` ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/bridge/br_arp_nd_proxy.c | 8 +++++---
net/bridge/br_fdb.c | 28 ++++++++++++++++++----------
2 files changed, 23 insertions(+), 13 deletions(-)

diff --git a/net/bridge/br_arp_nd_proxy.c b/net/bridge/br_arp_nd_proxy.c
index f507437e784f7..1914027e8bb3c 100644
--- a/net/bridge/br_arp_nd_proxy.c
+++ b/net/bridge/br_arp_nd_proxy.c
@@ -199,11 +199,12 @@ void br_do_proxy_suppress_arp(struct sk_buff *skb, struct net_bridge *br,

f = br_fdb_find_rcu(br, n->ha, vid);
if (f) {
+ const struct net_bridge_port *dst = READ_ONCE(f->dst);
bool replied = false;

if ((p && (p->flags & BR_PROXYARP)) ||
- (f->dst && (f->dst->flags & (BR_PROXYARP_WIFI |
- BR_NEIGH_SUPPRESS)))) {
+ (dst && (dst->flags & (BR_PROXYARP_WIFI |
+ BR_NEIGH_SUPPRESS)))) {
if (!vid)
br_arp_send(br, p, skb->dev, sip, tip,
sha, n->ha, sha, 0, 0);
@@ -463,9 +464,10 @@ void br_do_suppress_nd(struct sk_buff *skb, struct net_bridge *br,

f = br_fdb_find_rcu(br, n->ha, vid);
if (f) {
+ const struct net_bridge_port *dst = READ_ONCE(f->dst);
bool replied = false;

- if (f->dst && (f->dst->flags & BR_NEIGH_SUPPRESS)) {
+ if (dst && (dst->flags & BR_NEIGH_SUPPRESS)) {
if (vid != 0)
br_nd_send(br, p, skb, n,
skb->vlan_proto,
diff --git a/net/bridge/br_fdb.c b/net/bridge/br_fdb.c
index 882b6a67e11f8..10737ffffd7d0 100644
--- a/net/bridge/br_fdb.c
+++ b/net/bridge/br_fdb.c
@@ -239,6 +239,7 @@ struct net_device *br_fdb_find_port(const struct net_device *br_dev,
const unsigned char *addr,
__u16 vid)
{
+ const struct net_bridge_port *dst;
struct net_bridge_fdb_entry *f;
struct net_device *dev = NULL;
struct net_bridge *br;
@@ -251,8 +252,11 @@ struct net_device *br_fdb_find_port(const struct net_device *br_dev,
br = netdev_priv(br_dev);
rcu_read_lock();
f = br_fdb_find_rcu(br, addr, vid);
- if (f && f->dst)
- dev = f->dst->dev;
+ if (f) {
+ dst = READ_ONCE(f->dst);
+ if (dst)
+ dev = dst->dev;
+ }
rcu_read_unlock();

return dev;
@@ -342,7 +346,7 @@ static void fdb_delete_local(struct net_bridge *br,
vg = nbp_vlan_group(op);
if (op != p && ether_addr_equal(op->dev->dev_addr, addr) &&
(!vid || br_vlan_find(vg, vid))) {
- f->dst = op;
+ WRITE_ONCE(f->dst, op);
clear_bit(BR_FDB_ADDED_BY_USER, &f->flags);
return;
}
@@ -353,7 +357,7 @@ static void fdb_delete_local(struct net_bridge *br,
/* Maybe bridge device has same hw addr? */
if (p && ether_addr_equal(br->dev->dev_addr, addr) &&
(!vid || (v && br_vlan_should_use(v)))) {
- f->dst = NULL;
+ WRITE_ONCE(f->dst, NULL);
clear_bit(BR_FDB_ADDED_BY_USER, &f->flags);
return;
}
@@ -783,6 +787,7 @@ int br_fdb_test_addr(struct net_device *dev, unsigned char *addr)
int br_fdb_fillbuf(struct net_bridge *br, void *buf,
unsigned long maxnum, unsigned long skip)
{
+ const struct net_bridge_port *dst;
struct net_bridge_fdb_entry *f;
struct __fdb_entry *fe = buf;
int num = 0;
@@ -798,7 +803,8 @@ int br_fdb_fillbuf(struct net_bridge *br, void *buf,
continue;

/* ignore pseudo entry for local MAC address */
- if (!f->dst)
+ dst = READ_ONCE(f->dst);
+ if (!dst)
continue;

if (skip) {
@@ -810,8 +816,8 @@ int br_fdb_fillbuf(struct net_bridge *br, void *buf,
memcpy(fe->mac_addr, f->key.addr.addr, ETH_ALEN);

/* due to ABI compat need to split into hi/lo */
- fe->port_no = f->dst->port_no;
- fe->port_hi = f->dst->port_no >> 8;
+ fe->port_no = dst->port_no;
+ fe->port_hi = dst->port_no >> 8;

fe->is_local = test_bit(BR_FDB_LOCAL, &f->flags);
if (!test_bit(BR_FDB_STATIC, &f->flags))
@@ -924,9 +930,11 @@ int br_fdb_dump(struct sk_buff *skb,

rcu_read_lock();
hlist_for_each_entry_rcu(f, &br->fdb_list, fdb_node) {
+ const struct net_bridge_port *dst = READ_ONCE(f->dst);
+
if (*idx < cb->args[2])
goto skip;
- if (filter_dev && (!f->dst || f->dst->dev != filter_dev)) {
+ if (filter_dev && (!dst || dst->dev != filter_dev)) {
if (filter_dev != dev)
goto skip;
/* !f->dst is a special case for bridge
@@ -934,10 +942,10 @@ int br_fdb_dump(struct sk_buff *skb,
* Therefore need a little more filtering
* we only want to dump the !f->dst case
*/
- if (f->dst)
+ if (dst)
goto skip;
}
- if (!filter_dev && f->dst)
+ if (!filter_dev && dst)
goto skip;

err = fdb_fill_info(skb, br, f,
--
2.53.0