Re: [PATCH] livepatch: add load/unload hooks to objects

[Christopher Arges <chris.j.arges@canonical.com>]

On Tue, Aug 30, 2016 at 11:41:28AM +0200, Jiri Kosina wrote:
> On Mon, 29 Aug 2016, Christopher Arges wrote:
> 
> > Another example is CVE-2016-2117. Here we need to unset NETIF_F_SG on a 
> > particular device. If the device is already loaded we need a way to 
> > fixup hw_features on an already allocated network device. Again this 
> > could be done in the init code of the patch, but a nicer solution would 
> > be to do this on a load/unload hook appropriately.
> 
> I am afraid this is more complicated than what you describe. You can't 
> just unset NETIF_F_SG and be done with it; look for example what might 
> happen if you clear the flag while skb_segment() is running and gcc is 
> refetching netdev_features_t (there is no READ_ONCE() for that). The same 
> holds for __ip6_append_data().
> I am not saying this can't be worked around, but it's way much more 
> complicated than just clearing a bit in a callback.
> 
> -- 
> Jiri Kosina
> SUSE Labs
>

Jiri,

Yes this example was meant more for showing how something like a load/unload
hook could make patching certain situations easier for a patch author.
Essentially it would be nice to have a place to run code right before patching,
without having to write an additional notifier for module load events.

In this specific example, for safety of setting hw_features perhaps one could
check if a set of functions are on the stacks of any tasks before executing
these hooks. Or ignore any skbs that are already in flight.

--chris