Re: [PATCH] net: sched: fix possible OOB write in fl_set_geneve_opt()

[Hangyu Hua]

On 30/5/2023 19:36, Simon Horman wrote:
> [Updated Pieter's email address, dropped old email address of mine]
>
> On Mon, May 29, 2023 at 12:36:15PM +0800, Hangyu Hua wrote:
> > If we send two TCA_FLOWER_KEY_ENC_OPTS_GENEVE packets and their total
> > size is 252 bytes(key->enc_opts.len = 252) then
> > key->enc_opts.len = opt->length = data_len / 4 = 0 when the third
> > TCA_FLOWER_KEY_ENC_OPTS_GENEVE packet enters fl_set_geneve_opt. This
> > bypasses the next bounds check and results in an out-of-bounds.
> >
> > Fixes: 0a6e77784f49 ("net/sched: allow flower to match tunnel options")
> > Signed-off-by: Hangyu Hua <hbh25y@xxxxxxxxx>
>
> Hi Hangyu Hua,
>
> Thanks. I think I see the problem too.
> But I do wonder, is this more general than Geneve options?
> That is, can this occur with any sequence of options, that
> consume space in enc_opts (configured in fl_set_key()) that
> in total are more than 256 bytes?

I think you are right. It is a good idea to add check in 
fl_set_vxlan_opt and fl_set_erspan_opt and fl_set_gtp_opt too.
But they should be submitted as other patches. fl_set_geneve_opt has 
already check this with the following code:

static int fl_set_geneve_opt(const struct nlattr *nla, struct 
fl_flow_key *key,
			     int depth, int option_len,
			     struct netlink_ext_ack *extack)
{
...
		if (new_len > FLOW_DIS_TUN_OPTS_MAX) {
			NL_SET_ERR_MSG(extack, "Tunnel options exceeds max size");
			return -ERANGE;
		}
...
}

This bug will only be triggered under this special 
condition(key->enc_opts.len = 252). So I think it will be better 
understood by submitting this patch independently.

By the way, I think memset's third param should be option_len in 
fl_set_vxlan_opt and fl_set_erspan_opt. Do I need to submit another 
patch to fix all these issues?

Thanks,
Hangyu

> > ---
> >   net/sched/cls_flower.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/net/sched/cls_flower.c b/net/sched/cls_flower.c
> > index e960a46b0520..a326fbfe4339 100644
> > --- a/net/sched/cls_flower.c
> > +++ b/net/sched/cls_flower.c
> > @@ -1153,6 +1153,9 @@ static int fl_set_geneve_opt(const struct nlattr *nla, struct fl_flow_key *key,
> >   	if (option_len > sizeof(struct geneve_opt))
> >   		data_len = option_len - sizeof(struct geneve_opt);
> >   
> > +	if (key->enc_opts.len > FLOW_DIS_TUN_OPTS_MAX - 4)
> > +		return -ERANGE;
> > +
> >   	opt = (struct geneve_opt *)&key->enc_opts.data[key->enc_opts.len];
> >   	memset(opt, 0xff, option_len);
> >   	opt->length = data_len / 4;
> > --
> > 2.34.1