Re: [PATCH v3 2/2] Make num_actions unsigned

[Pablo Neira Ayuso]

On Thu, Sep 28, 2023 at 07:55:09PM -0700, Joao Moreira wrote:
> On 2023-09-28 06:43, Pablo Neira Ayuso wrote:
> > On Wed, Sep 27, 2023 at 09:47:15AM -0700, joao@xxxxxxxxxxxxxxxxxx wrote:
> > > From: Joao Moreira <joao.moreira@xxxxxxxxx>
> > > 
> > > Currently, in nft_flow_rule_create function, num_actions is a signed
> > > integer. Yet, it is processed within a loop which increments its
> > > value. To prevent an overflow from occurring, make it unsigned and
> > > also check if it reaches 256 when being incremented.
> > > 
> > > Accordingly to discussions around v2, 256 actions are more than enough
> > > for the frontend actions.
> > > 
> > > After checking with maintainers, it was mentioned that front-end will
> > > cap the num_actions value and that it is not possible to reach such
> > > condition for an overflow. Yet, for correctness, it is still better to
> > > fix this.
> > > 
> > > This issue was observed by the commit author while reviewing a
> > > write-up
> > > regarding a CVE within the same subsystem [1].
> > > 
> > > 1 - https://nickgregory.me/post/2022/03/12/cve-2022-25636/
> > 
> > Yes, but this is not related to the netfilter subsystem itself, this
> > harderning is good to have for the flow offload infrastructure in
> > general.
> 
> Right, I'll try to look up where this would fit in then. I'm not an expert
> in the subsystem at all, so should take a minute or two for me to get to it
> and send a v4.

Thanks.

> > >  	struct nft_expr *expr;
> > > 
> > >  	expr = nft_expr_first(rule);
> > > @@ -99,6 +100,10 @@ struct nft_flow_rule
> > > *nft_flow_rule_create(struct net *net,
> > >  		    expr->ops->offload_action(expr))
> > >  			num_actions++;
> > > 
> > > +		/* 2^8 is enough for frontend actions, avoid overflow */
> > > +		if (num_actions == 256)
> > 
> > This cap is not specific of nf_tables, it should apply to all other
> > subsystems. This is the wrong spot.
> 
> Any pointers regarding where I should look at?

See flow_rule_alloc().