Subject: Re: [PATCHv3 net-next 2/3] bonding: Add ESN support to IPSec HW offload

From: Sabrina Dubroca <[email-masked]>
Date: Thu, 22 Aug 2024 10:39:11 +0200

2024-08-22, 09:10:47 +0200, Steffen Klassert wrote:
> On Thu, Aug 22, 2024 at 08:33:17AM +0800, Hangbin Liu wrote:
> > On Wed, Aug 21, 2024 at 03:39:48PM +0200, Sabrina Dubroca wrote:
> > > > > > > +	if (!real_dev->xfrmdev_ops ||
> > > > > > > +	    !real_dev->xfrmdev_ops->xdo_dev_state_advance_esn) {
> > > > > > > +		pr_warn("%s: %s doesn't support xdo_dev_state_advance_esn\n", __func__, real_dev->name);
> > > > > > 
> > > > > > xdo_dev_state_advance_esn is called on the receive path for every
> > > > > > packet when ESN is enabled (xfrm_input -> xfrm_replay_advance ->
> > > > > > xfrm_replay_advance_esn -> xfrm_dev_state_advance_esn), this needs to
> > > > > > be ratelimited.
> > > > > 
> > > > > How does xfrm_state offload work on bonding?
> > > > > Does every slave have its own negotiated SA?
> > > > 
> > > > Yes and no. Bonding only supports xfrm offload with active-backup mode. So only
> > > > current active slave keep the SA. When active slave changes, the sa on
> > > > previous slave is deleted and re-added on new active slave.
> > > 
> > > It's the same SA, there's no DELSA+NEWSA when we change the active
> > > slave (but we call xdo_dev_state_delete/xdo_dev_state_add to inform
> > > the driver/HW), and only a single NEWSA to install the offloaded SA on
> > > the bond device (which calls the active slave's xdo_dev_state_add).
> > 
> > Yes, thanks for the clarification. The SA is not changed, we just delete it
> > on old active slave
> > 
> > slave->dev->xfrmdev_ops->xdo_dev_state_delete(ipsec->xs);
> > 
> > And add to now one.
> > 
> > ipsec->xs->xso.real_dev = slave->dev;
> > slave->dev->xfrmdev_ops->xdo_dev_state_add(ipsec->xs, NULL)
> 
> Using the same key on two different devices is very dangerous.

It's only used by one device at a time, we only support offload with
"active-backup" mode, where only the current active slave can send
packets.

> Counter mode algorithms have the requirement that the IV
> must not repeat. If you use the same key on two devices,
> you can't guarantee that. If both devices use an internal
> counter (initialized to one) to generate the IV, then the
> IV repeats for the mumber of packets that were already
> sent on the old device. The algorithm is cryptographically
> broken in that case.

Aren't they basing the IV on the sequence number filled in the header?
If not, then I guess this stuff has been broken since 2020 :(
(18cb261afd7b ("bonding: support hardware encryption offload to slaves"))

> Instead of moving the existing state, it is better to
> request a rekey. Maybe by setting the old state to
> 'expired' and then send a km_state_expired() message.

But then you're going to drop packets during the whole rekey?

-- 
Sabrina