From: Zhengchuan Liang Date: Mon, 13 Apr 2026 09:08:46 +0000 (+0800) Subject: net: bridge: use a stable FDB dst snapshot in RCU readers X-Git-Tag: v7.1-rc1~36^2~73 X-Git-Url: http://git.ipfire.org/gitweb/?a=commitdiff_plain;h=df4601653201de21b487c3e7fffd464790cab808;p=thirdparty%2Fkernel%2Flinux.git 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 Reported-by: Juefei Pu Co-developed-by: Yuan Tan Signed-off-by: Yuan Tan Suggested-by: Xin Liu Tested-by: Ren Wei Signed-off-by: Zhengchuan Liang Signed-off-by: Ren Wei Reviewed-by: Ido Schimmel Acked-by: Nikolay Aleksandrov Link: https://patch.msgid.link/6570fabb85ecadb8baaf019efe856f407711c7b9.1776043229.git.zcliangcn@gmail.com Signed-off-by: Paolo Abeni --- 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,