Re: [PATCH net v4 1/2] bonding: fix null-ptr-deref in bond_rr_gen_slave_id()

[Jay Vosburgh <jv@jvosburgh.net>]

Nikolay Aleksandrov <razor@blackwall.org> wrote:

>On Wed, Mar 04, 2026 at 03:42:57PM +0800, Jiayuan Chen wrote:
>> From: Jiayuan Chen <jiayuan.chen@shopee.com>
>> 
>> 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 <jiayuan.chen@shopee.com>
>> ---
>>  drivers/net/bonding/bond_main.c | 19 +++++++++++++------
>>  1 file changed, 13 insertions(+), 6 deletions(-)
>> 
>
>IMO it's not worth it to waste memory in all modes, for an unpopular mode.
>I think it'd be better to add a null check in bond_rr_gen_slave_id(),
>READ/WRITE_ONCE() should be enough since it is allocated only once, and
>freed when the xmit code cannot be reachable anymore (otherwise we'd have
>more bugs now). The branch will be successfully predicted practically always,
>and you can also mark the ptr being null as unlikely. That way only RR takes
>a very minimal hit, if any.

	Is what you're suggesting different from Jiayuan's proposal[0],
in the sense of needing barriers in the XDP hot path to insure ordering?

	If I understand correctly, your suggestion is something like
(totally untested):

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index eb27cacc26d7..ac2a4fc0aad0 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -4273,13 +4273,17 @@ void bond_work_cancel_all(struct bonding *bond)
 static int bond_open(struct net_device *bond_dev)
 {
 	struct bonding *bond = netdev_priv(bond_dev);
+	u32 __percpu *rr_tx_tmp;
 	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)
+	if (BOND_MODE(bond) == BOND_MODE_ROUNDROBIN &&
+	    !READ_ONCE(bond->rr_tx_counter)) {
+		rr_tx_tmp = alloc_percpu(u32);
+		if (!rr_tx_tmp)
 			return -ENOMEM;
+		WRITE_ONCE(bond->rr_tx_counter, rr_tx_tmp);
+
 	}
 
 	/* reset slave->backup and slave->inactive */
@@ -4866,6 +4870,9 @@ static u32 bond_rr_gen_slave_id(struct bonding *bond)
 	struct reciprocal_value reciprocal_packets_per_slave;
 	int packets_per_slave = bond->params.packets_per_slave;
 
+	if (!READ_ONCE(bond->rr_tx_counter))
+		packets_per_slave = 0;
+
 	switch (packets_per_slave) {
 	case 0:
 		slave_id = get_random_u32();

	-J


[0] https://lore.kernel.org/netdev/e4a2a652784ec206728eb3a929a9892238c61f06@linux.dev/

---
	-Jay Vosburgh, jv@jvosburgh.net