Re: [PATCH net v7 1/2] net, bpf: fix null-ptr-deref in xdp_master_redirect() for down master

[Paolo Abeni]

On 4/11/26 2:55 AM, Jiayuan Chen wrote:
> syzkaller reported a kernel panic in bond_rr_gen_slave_id() reached via
> xdp_master_redirect(). Full decoded trace:
> 
>   https://syzkaller.appspot.com/bug?extid=80e046b8da2820b6ba73
> 
> bond_rr_gen_slave_id() dereferences bond->rr_tx_counter, a per-CPU
> counter that bonding only allocates in bond_open() when the mode is
> round-robin. If the bond device was never brought up, rr_tx_counter
> stays NULL.
> 
> The XDP redirect path can still reach that code on a bond that was
> never opened: bpf_master_redirect_enabled_key is a global static key,
> so as soon as any bond device has native XDP attached, the
> XDP_TX -> xdp_master_redirect() interception is enabled for every
> slave system-wide. The path xdp_master_redirect() ->
> bond_xdp_get_xmit_slave() -> bond_xdp_xmit_roundrobin_slave_get() ->
> bond_rr_gen_slave_id() then runs against a bond that has no
> rr_tx_counter and crashes.
> 
> Fix this in the generic xdp_master_redirect() by refusing to call into
> the master's ->ndo_xdp_get_xmit_slave() when the master device is not
> up. IFF_UP is only set after ->ndo_open() has successfully returned,
> so this reliably excludes masters whose XDP state has not been fully
> initialized. Drop the frame with XDP_ABORTED so the exception is
> visible via trace_xdp_exception() rather than silently falling through.
> This is not specific to bonding: any current or future master that
> defers XDP state allocation to ->ndo_open() is protected.
> 
> Fixes: 879af96ffd72 ("net, core: Add support for XDP redirection to slave device")
> Reported-by: syzbot+80e046b8da2820b6ba73@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/698f84c6.a70a0220.2c38d7.00cc.GAE@xxxxxxxxxx/T/
> Suggested-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Acked-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>
> ---
>  net/core/filter.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index cf2113af4bc9..9ec70c4b7723 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -4398,6 +4398,8 @@ u32 xdp_master_redirect(struct xdp_buff *xdp)
>  	struct net_device *master, *slave;
>  
>  	master = netdev_master_upper_dev_get_rcu(xdp->rxq->dev);
> +	if (unlikely(!(master->flags & IFF_UP)))
> +		return XDP_ABORTED;
>  	slave = master->netdev_ops->ndo_xdp_get_xmit_slave(master, xdp);

The AI review noted that the master could be (theoretically) NULL here.
Since that event is not a regression (unconditional `master` dereference
already present) and syzkaller failed to trigger it (despite being the
sort of thing syzkaller is very good to find) make me thing it's better
to eventually follow-up instead requesting another revision here, but
please have a look.

Thanks,

Paolo

>  	if (slave && slave != xdp->rxq->dev) {
>  		/* The target device is different from the receiving device, so