Subject: Re: [PATCH net v3 1/2] bonding: fix null-ptr-deref in bond_rr_gen_slave_id()

From: Jay Vosburgh <[email-masked]>
Date: Fri, 27 Feb 2026 19:01:47 -0800

Jiayuan Chen <[email-masked]> wrote:

>From: Jiayuan Chen <[email-masked]>
>
>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.
>
>The normal TX path (bond_xmit_roundrobin) is not affected because TX
>requires the bond to be UP, which guarantees rr_tx_counter is allocated.
>However, bond_xmit_get_slave() (ndo_get_xmit_slave) has the same code
>pattern via bond_xmit_roundrobin_slave_get() and could theoretically
>hit the same issue.

	As a practical matter, though, I don't think the
ndo_get_xmit_slave path can actually hit the issue, as that looks to
only be called from Infiniband, which is only supported in bonding for
active-backup mode.

>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.
>
>Fixes: 879af96ffd72 ("net, core: Add support for XDP redirection to slave device")
>Reported-by: [email-masked]
>Closes: https://lore.kernel.org/all/[email-masked]/T/
>Signed-off-by: Jiayuan Chen <[email-masked]>

	My only concern is that this will waste a percpu u32 per bond
device for the majority of bonding use cases (which use modes other than
balance-rr), which could be a few hundred bytes on a large machine.

	Does everything work reliably if the rr_tx_counter allocation
happens conditionally on mode == BOND_MODE_ROUNDROBIN in bond_setup, as
well as in bond_option_mode_set?

	-J

>---
> drivers/net/bonding/bond_main.c | 12 ++++++------
> 1 file changed, 6 insertions(+), 6 deletions(-)
>
>diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
>index 78cff904cdc3..9f63f67d8418 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 +6405,12 @@ static int bond_init(struct net_device *bond_dev)
> 	if (!bond->wq)
> 		return -ENOMEM;
> 
>+	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
>

---
	-Jay Vosburgh, [email-masked]