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

[Joao Moreira]

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.

> > Signed-off-by: Joao Moreira <joao.moreira@xxxxxxxxx>
> > ---
> >  net/netfilter/nf_tables_offload.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/netfilter/nf_tables_offload.c 
> > b/net/netfilter/nf_tables_offload.c
> > index 12ab78fa5d84..9a86db1f0e07 100644
> > --- a/net/netfilter/nf_tables_offload.c
> > +++ b/net/netfilter/nf_tables_offload.c
> > @@ -90,7 +90,8 @@ struct nft_flow_rule *nft_flow_rule_create(struct 
> > net *net,
> >  {
> >  	struct nft_offload_ctx *ctx;
> >  	struct nft_flow_rule *flow;
> > -	int num_actions = 0, err;
> > +	unsigned int num_actions = 0;
> > +	int err;
>
> reverse xmas tree.

ack.

> >  	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?

> Moreover, please, add a definition for this, no hardcoded values.

Ack.

> > +			return ERR_PTR(-ENOMEM);
>
> Better E2BIG or similar, otherwise this propagates to userspace as
> ENOMEM.

Ack.

> > +
> >  		expr = nft_expr_next(expr);
> >  	}
> >
> > --
> > 2.42.0