* FAILED: patch "[PATCH] net: bridge: use a stable FDB dst snapshot in RCU readers" failed to apply to 6.6-stable tree
@ 2026-05-01 12:11 gregkh
2026-05-04 7:47 ` [PATCH 6.6.y] net: bridge: use a stable FDB dst snapshot in RCU readers Sasha Levin
0 siblings, 1 reply; 2+ messages in thread
From: gregkh @ 2026-05-01 12:11 UTC (permalink / raw)
To: zcliangcn, bird, enjou1224z, idosch, n05ec, pabeni, razor,
tomapufckgml, yifanwucs, yuantan098
Cc: stable
The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.
To reproduce the conflict and resubmit, you may use the following commands:
git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x df4601653201de21b487c3e7fffd464790cab808
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026050100-suave-boaster-8c68@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..
Possible dependencies:
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From df4601653201de21b487c3e7fffd464790cab808 Mon Sep 17 00:00:00 2001
From: Zhengchuan Liang <zcliangcn@gmail.com>
Date: Mon, 13 Apr 2026 17:08:46 +0800
Subject: [PATCH] net: bridge: use a stable FDB dst snapshot in RCU readers
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>
diff --git a/net/bridge/br_arp_nd_proxy.c b/net/bridge/br_arp_nd_proxy.c
index 0c8a06cdd46f..deb1ab1f24b0 100644
--- a/net/bridge/br_arp_nd_proxy.c
+++ b/net/bridge/br_arp_nd_proxy.c
@@ -201,11 +201,12 @@ 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_is_neigh_suppress_enabled(f->dst, vid)) {
+ (dst && (dst->flags & BR_PROXYARP_WIFI)) ||
+ br_is_neigh_suppress_enabled(dst, vid)) {
if (!vid)
br_arp_send(br, p, skb->dev, sip, tip,
sha, n->ha, sha, 0, 0);
@@ -469,9 +470,10 @@ 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 (br_is_neigh_suppress_enabled(f->dst, vid)) {
+ if (br_is_neigh_suppress_enabled(dst, vid)) {
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 e2c17f620f00..6eb3ab69a514 100644
--- a/net/bridge/br_fdb.c
+++ b/net/bridge/br_fdb.c
@@ -236,6 +236,7 @@ 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;
@@ -248,8 +249,11 @@ 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;
@@ -346,7 +350,7 @@ 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;
}
@@ -357,7 +361,7 @@ 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;
}
@@ -928,6 +932,7 @@ 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;
unsigned long delta;
@@ -944,7 +949,8 @@ 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) {
@@ -956,8 +962,8 @@ 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)) {
@@ -1083,9 +1089,11 @@ 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 < ctx->fdb_idx)
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
@@ -1093,10 +1101,10 @@ 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,
^ permalink raw reply related [flat|nested] 2+ messages in thread
* [PATCH 6.6.y] net: bridge: use a stable FDB dst snapshot in RCU readers
2026-05-01 12:11 FAILED: patch "[PATCH] net: bridge: use a stable FDB dst snapshot in RCU readers" failed to apply to 6.6-stable tree gregkh
@ 2026-05-04 7:47 ` Sasha Levin
0 siblings, 0 replies; 2+ messages in thread
From: Sasha Levin @ 2026-05-04 7:47 UTC (permalink / raw)
To: stable
Cc: Zhengchuan Liang, stable, Yifan Wu, Juefei Pu, Yuan Tan, Xin Liu,
Ren Wei, Ren Wei, Ido Schimmel, Nikolay Aleksandrov, Paolo Abeni,
Sasha Levin
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 `*idx < cb->args[2]` instead of `*idx < 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 f033a51675602..985aaf7ff1564 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_is_neigh_suppress_enabled(f->dst, vid)) {
+ (dst && (dst->flags & BR_PROXYARP_WIFI)) ||
+ br_is_neigh_suppress_enabled(dst, vid)) {
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 (br_is_neigh_suppress_enabled(f->dst, vid)) {
+ if (br_is_neigh_suppress_enabled(dst, vid)) {
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 fa2970db21301..0a51f648c57ec 100644
--- a/net/bridge/br_fdb.c
+++ b/net/bridge/br_fdb.c
@@ -246,6 +246,7 @@ 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;
@@ -258,8 +259,11 @@ 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;
@@ -349,7 +353,7 @@ 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;
}
@@ -360,7 +364,7 @@ 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;
}
@@ -790,6 +794,7 @@ 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;
unsigned long delta;
@@ -806,7 +811,8 @@ 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) {
@@ -818,8 +824,8 @@ 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)) {
@@ -940,9 +946,11 @@ 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
@@ -950,10 +958,10 @@ 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
^ permalink raw reply related [flat|nested] 2+ messages in thread
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2026-05-01 12:11 FAILED: patch "[PATCH] net: bridge: use a stable FDB dst snapshot in RCU readers" failed to apply to 6.6-stable tree gregkh
2026-05-04 7:47 ` [PATCH 6.6.y] net: bridge: use a stable FDB dst snapshot in RCU readers Sasha Levin
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