[PATCH net v5 1/2] bonding: fix null-ptr-deref in bond_rr_gen_slave_id()

[Jiayuan Chen <jiayuan.chen@linux.dev>]

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 adding a NULL check with unlikely() in bond_rr_gen_slave_id()
before dereferencing rr_tx_counter. When rr_tx_counter is NULL (bond was
never opened), fall back to get_random_u32() for slave selection. The
allocation in bond_open() is kept, with WRITE_ONCE() added to safely
publish the pointer to the XDP read side. A plain read suffices for the
!bond->rr_tx_counter guard in bond_open() itself, as bond_open() runs
under RTNL lock and is the only writer of rr_tx_counter.

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 | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 444519078da3..b8ec87625ce3 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -4290,9 +4290,11 @@ static int bond_open(struct net_device *bond_dev)
 	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)
+		u32 __percpu *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 */
@@ -4883,6 +4885,9 @@ 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 (unlikely(!READ_ONCE(bond->rr_tx_counter)))
+		return get_random_u32();
+
 	switch (packets_per_slave) {
 	case 0:
 		slave_id = get_random_u32();
-- 
2.43.0