Re: [PATCH net-next v5 02/10] net: sched: act_sample: add action cookie to sample

[Adrián Moreno]

On Wed, Jun 26, 2024 at 04:28:01PM GMT, Eelco Chaudron wrote:
>
>
> On 25 Jun 2024, at 22:51, Adrian Moreno wrote:
>
> > If the action has a user_cookie, pass it along to the sample so it can
> > be easily identified.
> >
> > Signed-off-by: Adrian Moreno <amorenoz@xxxxxxxxxx>
> > ---
> >  net/sched/act_sample.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> >
> > diff --git a/net/sched/act_sample.c b/net/sched/act_sample.c
> > index a69b53d54039..2ceb4d141b71 100644
> > --- a/net/sched/act_sample.c
> > +++ b/net/sched/act_sample.c
> > @@ -167,7 +167,9 @@ TC_INDIRECT_SCOPE int tcf_sample_act(struct sk_buff *skb,
> >  {
> >  	struct tcf_sample *s = to_sample(a);
> >  	struct psample_group *psample_group;
> > +	u8 cookie_data[TC_COOKIE_MAX_SIZE];
> >  	struct psample_metadata md = {};
> > +	struct tc_cookie *user_cookie;
> >  	int retval;
> >
> >  	tcf_lastuse_update(&s->tcf_tm);
> > @@ -189,6 +191,16 @@ TC_INDIRECT_SCOPE int tcf_sample_act(struct sk_buff *skb,
> >  		if (skb_at_tc_ingress(skb) && tcf_sample_dev_ok_push(skb->dev))
> >  			skb_push(skb, skb->mac_len);
> >
> > +		rcu_read_lock();
> > +		user_cookie = rcu_dereference(a->user_cookie);
> > +		if (user_cookie) {
> > +			memcpy(cookie_data, user_cookie->data,
> > +			       user_cookie->len);
>
> Maybe I’m over paranoid, but can we assume user_cookie->len, will not be larger than TC_COOKIE_MAX_SIZE?
> Or should we do something like min(user_cookie->len, sizeof(cookie_data))
>

I think it's good to be paranoid with this kind of things. I do,
however, think it should be safe to use. The cookie is extracted from
the netlink attribute directly and its length is verified with the
nla_policy [1]. So nothing that comes into the kernel should be larger
than TC_COOKIE_MAX_SIZE.

I guess if there is some previous bug that allows for the size to get
corrupted, then this might happen but doing those kind of checks in the
fast path seems a bit excessive. For example, Ilya argued in v2 [2] that
we should avoid zeroing "u8 cookie_data[TC_COOKIE_MAX_SIZE]" to safe the
unneeded cycles.

[1] https://github.com/torvalds/linux/blob/55027e689933ba2e64f3d245fb1ff185b3e7fc81/net/sched/act_api.c#L1299
[2] https://patchwork.kernel.org/project/netdevbpf/patch/20240603185647.2310748-3-amorenoz@xxxxxxxxxx/

Thanks.
Adrián

> > +			md.user_cookie = cookie_data;
> > +			md.user_cookie_len = user_cookie->len;
> > +		}
> > +		rcu_read_unlock();
> > +
> >  		md.trunc_size = s->truncate ? s->trunc_size : skb->len;
> >  		psample_sample_packet(psample_group, skb, s->rate, &md);
> >
> > --
> > 2.45.1
>