From: Jiayuan Chen bond_rr_gen_slave_id() dereferences bond->rr_tx_counter without a NULL check. rr_tx_counter is a per-CPU counter only allocated in bond_open() when the bond mode is round-robin. If the bond device was never brought up, rr_tx_counter remains NULL, causing a null-ptr-deref. The XDP redirect path can reach this code even when the bond is not up: bpf_master_redirect_enabled_key is a global static key, so when any bond device has native XDP attached, the XDP_TX -> xdp_master_redirect() interception is enabled for all bond slaves system-wide. This allows the path xdp_master_redirect() -> bond_xdp_get_xmit_slave() -> bond_xdp_xmit_roundrobin_slave_get() -> bond_rr_gen_slave_id() to be reached on a bond that was never opened. Fix this by allocating rr_tx_counter unconditionally in bond_init() (ndo_init), which is called by register_netdevice() and covers both device creation paths (bond_create() and bond_newlink()). This also handles the case where bond mode is changed to round-robin after device creation. The conditional allocation in bond_open() is removed. Since bond_destructor() already unconditionally calls free_percpu(bond->rr_tx_counter), the lifecycle is clean: allocate at ndo_init, free at destructor. Note: rr_tx_counter is only used by round-robin mode, so this deliberately allocates a per-cpu u32 that goes unused for other modes. Conditional allocation (e.g., in bond_option_mode_set) was considered but rejected: the XDP path can race with mode changes on a downed bond, and adding memory barriers to the XDP hot path is not justified for saving 4 bytes per CPU. Fixes: 879af96ffd72 ("net, core: Add support for XDP redirection to slave device") Reported-by: syzbot+80e046b8da2820b6ba73@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/698f84c6.a70a0220.2c38d7.00cc.GAE@google.com/T/ Signed-off-by: Jiayuan Chen --- drivers/net/bonding/bond_main.c | 19 +++++++++++++------ 1 file changed, 13 insertions(+), 6 deletions(-) diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c index 78cff904cdc3..55b5c7a6cb5f 100644 --- a/drivers/net/bonding/bond_main.c +++ b/drivers/net/bonding/bond_main.c @@ -4279,12 +4279,6 @@ static int bond_open(struct net_device *bond_dev) struct list_head *iter; struct slave *slave; - if (BOND_MODE(bond) == BOND_MODE_ROUNDROBIN && !bond->rr_tx_counter) { - bond->rr_tx_counter = alloc_percpu(u32); - if (!bond->rr_tx_counter) - return -ENOMEM; - } - /* reset slave->backup and slave->inactive */ if (bond_has_slaves(bond)) { bond_for_each_slave(bond, slave, iter) { @@ -6411,6 +6405,19 @@ static int bond_init(struct net_device *bond_dev) if (!bond->wq) return -ENOMEM; + /* rr_tx_counter is only used in round-robin mode, but we allocate + * it unconditionally because the XDP redirect path + * (xdp_master_redirect -> bond_xdp_get_xmit_slave) can reach here + * even when the bond is not up, and deferring allocation to + * bond_open or bond_option_mode_set would require memory barriers + * on the XDP hot path. The cost is a per-cpu u32 per bond device. + */ + bond->rr_tx_counter = alloc_percpu(u32); + if (!bond->rr_tx_counter) { + destroy_workqueue(bond->wq); + return -ENOMEM; + } + bond->notifier_ctx = false; spin_lock_init(&bond->stats_lock); -- 2.43.0