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

[Nikolay Aleksandrov <razor@blackwall.org>]

On Wed, Mar 04, 2026 at 09:27:28AM -0800, Jay Vosburgh wrote:
> 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):
> 

Basically yes, that is what I'm proposing + an unlikely() around that
null check since it is really unlikely and will be always predicted
correctly, this way it's only for RR mode.

> 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